7.20 and the subsequent patches have had a few big changes that I feel push Puck further into the support role, both by nerfing your midlane power and by buffing your supporting power
Number 1 is that Dream Coil now stops the use of TP's, which means you no longer need to wait for them to start the TP before using coil to cancel it. This makes your ult a lot more useful in ganks, and makes it easier to secure kills on one or two targets after you hit 6.
Number 2 is the changes to items, such as Null Talisman alongside Wraith Band and Bracer. The new Bracer allows strength heroes to get magic resist into their builds easier, and wraith plus treads allows agi heroes to spike much earlier in the mid game, while null provides little to int heroes. It also stops the 2 early bracer -> late Atos build that Pugna, Ench and Puck could use to make them tankier in the midgame without slowing their item build significantly. QoP, SF, Storm, Pug and Puck all feel much weaker in the mid lane, and struggle to make much of an impact, so it seems like supporting is still going to be the way to go for Puck, and probably QoP if you can make her work. New item Holy Locket is basically a cheaper pipe, and once again it makes it much more difficult to scale into mid/late with magical nukers.
Final changes are about the meta shifts: Phantom Assassin and Slark were insta picks in 7.20, and while they've been toned down a bit they're still really strong. However Puck is probably the best support to counter them due to Phase Shift. PA Blink now gives her bonus AS for 2 seconds, which means if you use Phase you can dodge her entire window to kill you and get out alive. Dream Coil is also able to hit her during Blur. If Slark pounces you, fire an Ilussory Orb towards the trees, and phase shift. Then jaunt to your orb, as his pounce will be on cooldown. If Slark ults, your coil stops him from running away, and your silence can stop him from using dark pact on dust of appearance, making him easier for your team to deal with.
-A lot of AOE damage
-Good at teamfighting
-Q gives strong jukes/repositioning tool
-Q gives vision, lets you scout rosh from a distance
-Hard to kill, perfect counter to Slark/Ursa/Space Cow
-Normally a mid, can bamboozle enemies into sub-optimal picks
-Needs good mechanics to stay alive
-Relies heavily on positioning
-BKB shuts you down completely
-Could tilt your team before the match begins
Right now Puck is in a bad place as a midlaner. Roaming to gank is harder, Puck doesn't do enough damage by itself to get consistent kills in the midgame, BKB completely nullifies your utility and damage, shrines make it harder to win lane by attrition, and if you go blink first you're often too squishy while veil first leaves you too exposed during ganks. All of these factors lead to less than 45% winrate against your best matchup icon=broodmother, and around 40% winrate overall.
So instead of going mid or demanding the frog release 7.20, I've focused on the fact that puck is good at trading early, has a massive AOE silence, and has a powerful teamfighting ultimate to put it into the support position. If you can land a good coil on multiple enemies it can turn the tide of a teamfight to your team's favour, and you can still have a midlaner who scales well into mid game, since by the time you start hitting talents Puck begins to fall behind the likes of QoP, who is better at level 10 due to her talents.
This switch to support leads to what may seem confusing, maxxing out on Waning Rift rather than Illusory Orb, with only a value point early on (for easier trading in lane, more mobility and for camp stacking). This is because the damage and mana cost of a max rift and a max orb are the same, but W max gives us a much more powerful AOE silence, which works better in 2v2, similar to how CM often prefers her Crystal Nova over Frostbite in a 2v2 lane, despite Frostbite doing more damage to a single target. It's also better at ganking without overexposing yourself, since pre-blink, Puck trades away all mobility for damage if you chose to max Q. This can lead to situations where you try to gank someone, but they were tankier than you thought, or had a wand and you end up dead. However, this doesn't happen if your damage is in your W instead, meaning you can save Q for mobility at the end of a gank, rather than being forced to use it at the start of one.
Level 2 Phase Shift lets you constantly trade without worrying about creep aggro, and lets you dodge all kinds of projectiles, from attacks to spells. Against someone like skywrath you can gain health every clash due to stick charges if you keep avoiding his bolts, and this constant trading is important. Puck's low health means against an extended fight you'll lose, so winning the battle of attrition is key. Keep whittling them down, so that if they commit to a fight with you they risk dying to your lane partner. If you have an enemy at <50% health and your ally has a stun, you can try to burst them down quickly, although try to avoid jumping in and using all your spells against full health opponents, since the longer a fight goes, the more likely it is you're dead, and if they're able to stand their ground you will die.
First two talents are the armour, especially since some of our later item pickups like hammer and atos will give up raw HP, and the spell amp for better teamfight damage. Level 20 is actually a difficult choice, since the -8s rift cooldown can sometimes be invaluable against someone like storm that needs spells to stay alive, and if you bought an orchid it is basically a death sentence for him. It also comboes well with meteor hammer if you go Waning Rift-> Dream Coil-> Meteor Hammer-> Waning Rift for constant AOE disables and damage. Mostly I find myself picking the orb speed/distance though, so it's easier to weave about in fights, and also so I can do things like checking rosh or pushing waves from further back, keeping myself safer, especially important once death timers start getting longer and longer. Level 25 you'll want the gold almost all the time. If you have a Wisp on your team and they took the attack tether talent, you can take rapid fire, but I wouldn't pick Puck and Wisp to try to pull off this combo, and unless you buy items like Maelstrom it won't do much damage, so this is probably something to try once or twice in turbo and then never again.
Starting items are courier, one ward, two sets of tangoes so you can pool with the midlaner, and a stick to turn into wand or a ring of protection to turn into urn, depending on if you're facing spell spam or auto attack harass.
If you don't need to give the mid regen, and if the other support bought the courier, then you can pickup a windlace, which will be useful later for either vessel or eul's, which makes your trading even better, especially against melee opponents.
Early on you'll want to get Urn of Shadows in order to make your ganks more powerful, Magic Wand to make yourself a little harder to kill, and boots to make yourself more mobile before blink. You can upgrade to either Arcane Boots (if you want Lotus Orb, or when allied with mana hungry guys like storm), or Tranquil Boots (better MS, and once you start building Eul's, Hammer or Orchid you never need to return to base)
Next you'll want a Blink Dagger as your core item, anything else is more of a nice-to-have than a need-to-have. Eul's Scepter of Divinity solves mana issues, and when comboed with your E makes you unkillable by carries like slark/ursa that enjoy jumping on squishy supports, or as a way to wait out a BKB since you do entirely magic damage. It also lets you break Linken's Sphere if you need to against storm/weaver etc., without wasting a charge of Vessel. Spirit Vessel is a good upgrade, the ms and hp are great to make you even harder to kill, although if it's often sitting at 0 charges don't feel forced into upgrading it, and it's often better to grab a hammer before upgrading unless you desperately need to stop a necro/slark. Meteor Hammer gives you HP, HP regen, and comboes with your ult for massive AOE stuns, all three things being great for the way we're playing Puck.
Lotus Orb is good if you need a dispel, builds out of arcanes and gives you some armour. The rest of the extension items are for lockdown and are mostly case-by-case, Orchid Malevolence is for an instant silence against someone like weaver, Rod of Atos makes you tankier, Sheepstick can stop carries getting off their BKB before you burst them down, Shiva's Guard makes your AOE even better, as well as armour and AS slow to make you tankier. Aghanim's Scepter is a good counter to BKB, but if you're against a carry like Monkey King that prefers to stand his ground with BKB + ult, then maybe don't get it, but some carries like PA and BS rely on their mobility, and Aghs can easily win you a fight if you catch them by surprise. BoT are great since you can easily shove waves by yourself and are incredibly difficult to catch, so sometimes picking them up and ratting is the best option, and so I often get them once I hit level 25 and have the income.
One item you may notice is missing is Veil of Discord. The extra damage isn't that good IMO, because Puck's greatest struggle is with spell immunity or getting burst by an enemy without using its spells, and extra magic damage doesn't help with either case. I much prefer tankier items to prevent being bursted, and to help wait out BKB duration, itemising around our shortcomings rather than building items that help in a snowball or against players that never build BKB. If you're against carries like a Slark/OD who don't buy BKB ever, or a carry like TB/PL where destroying illusions is needed then veil can work. It's just too situational that I rarely ever buy it, even when teamed with someone like Skywrath or Pugna.
-Q lets you stack two camps at once, or stack one camp from in the lane, which gives you a lot more gold in the midgame than most other supports.
-If you press E, all projectiles are instantly disjointed. So if running from a stun, tap E and instantly start moving again
-Your ultimate does a mini-stun on cast, then a second stun when the line breaks. So you can use your ultimate to cancel channels like TP's or Witch Doctor's and Crystal Maiden's ultimates
-blink dagger can be cast instantly from phase shift if you blink in the direction you were facing before phase shift. This lets you dodge radiance or other DOT effects that would cancel it.
-Level 25 can leave you with a lot of unreliable gold. If you have enough for buyback and 2k surplus, buy a Hyperstone, then a moonshard. Feeding it to your teammates lowers your networth, which means if you die you feed less gold to the enemy, and it's also a good way to help your team close out the game if you're having trouble taking rax and highground.
-Jaunt and Phase shift both don't proc effects like wand, or more importantly Arcane Curse and Last Word, letting you use them freely without worrying about the silence. This means you can actually dodge the damage from last word if you phase shift right as the silence is about to happen
Overall, Puck is a pretty good pick in most lanes. You aren't a weak laner like a Rubick, so you can help Spectre survive her early game, or have an Ursa and dominate the lane. Once your hit fights though, there are some good comobos:
Ancient Apparition: Land your ult, then he throws his. Even if the enemy try to run they still get stunned, so they always get hit by the blast. He also helps AOE magic nukers, and you have three skills to benefit from this.
Bloodseeker: Another AOE silence with Blood Rite, and one that works very well with your ult. Combining your silences together can lock down a huge area and stop enemy supports from doing anything. Make sure you check to see if he wants to carry an urn himself, as only one urn can gain charges. Your ult is able to cancel and prevent TP's, but due to the minute long cooldown I recommend you do this sparingly and get Eul's early on, as stopping TP's improves his gank success by about 500%.
Pudge: His hook breaker your ult, stunning the enemy and making it easier for him to follow up with his rot dismember combo. Your Q can also be used across trees to help him see juking enemies, but make sure you don't jump in front of his hook.
Legion Commander: Your ult stops them from running away, so she can always get a duel off. Once you buy Eul's you can also pop Linken's for her, without her needing to buy a halberd.
Ursa: biggest weakness is being kited, your ult stops that. He's also a good lane winner, and you are too
Kunnka: AOE stuns that combo with your silences and ult. Meteor Hammer becomes overkill in teamfights. His ship also delays half of all damage for a few seconds, so you become way tankier for a little while
And out of fights there are some good combos:
Alchemist: Use Q to double stack camps. Alch gets six slotted by min 20. you win. You can also keep people in acid with ult, and Q keeps vision if they try to juke his Unstable Concoction. He also gives you sceptre for free if the game goes late
Every squishy support: When warding you are much harder to kill, since you Q lets you see enemies and escape quickly, meaning it's harder for you to get caught out by a slark and get flamed for feeding than it is for a CM or Shaman. Make sure you're the one warding, and if you need to ask your buddy to buy wards and drop them in fountain or on courier.