Few things I've learned on choosing runes for supports

Certain Engage supports like Nautilus and Leona can deal crazy amounts of magic damage early with Aftershock. Make sure to pick Resolve as your secondary if you pick an AP support and grab the +5 MR. It will save you.

When tank supports like Braum and Taric are against Poke supports like Sona, Karma, Nami and Lulu get a lot out of Aery poke and deal magic damage. If you are not prepared to go all in get Guardian to help mitigate a lot of the poke damage they do. The MS boost will also allow you to retreat faster to get out of harass range. Early MR from Resolve and a quick MR buy can save you a lot of hassle too. Upgrading Relic can help with sustain a lot as well.

Aery can proc with ANY buff you give a champion. Nami E for example buffs an allies Auto attacks but with Aery it also give a shield now to prevent harassing damage in lane. This will also work for Braum W, Nunu W, Lulu W, Sona Q and E, Zilean E (not sure about Q) and many other abilities that champions can grant buffs to allies with.

Press the attack gives all ADC a 3 hit proc for lots of extra damage. Keep track of this as this will be when a big burst of damage comes in and you take 12% extra too boot. Time Exhaust for this 3 proc to mitigate a good chunk of this extra damage or try to keep the ADC switching between targets so they can't get 3 auto attacks off on one target.

Fleet Footwork is a great rune to take as an ADC when the enemy will be bullying you due to the sustain it gives. If you're against a Sona Cait or some other similar lane pick this rune to help you sustain through laning phase.

Kleptomancy is nice if you know you can bully the opponent and get free auto attacks. Otherwise it tends to be pretty lack luster on the support. It's amazing on Ezreal though.

Glacial Augment is a great rune if you need to stick to an opponent for a time. Tahm Kench and Taric get the most out of it but you will only be able to use this when against other tank supports as AP will kite you too hard to begin with. It is also nice for adding more peel to a melee support as you can apply that 40% slow with most of your auto attacks. I would say Tahm Kench gets the most out of this ruin as it makes it laughably easy to proc 3 stacks of passive with it. Just Q to slow them then walk up AA for slow and they are as good as eaten.

Manaflow Band is easily one of the best support runes as it gives you free mana every 60 seconds. You will rarely have mana issues with this. Combine with biscuit delivery and you will not ever run out of mana.

Transcendence is very powerful late, Celerity is very powerful early, Absolute Focus is great mid game. Pic the one that you feel will best fit your game. If you think you are scaling into late pick Transcendence. early pressure Celerity is nice for the extra MS to run people down with. Absolute Focus is pretty reliable as long as you keep your health up. An exception would be Nami as she always wants Celerity due to her passive.

Future Market seems nice in early game but if you're confident you can hold out for 10 minutes free boots with +10 extra MS is better. Don't forget boots are 300 to start plus it discounts upgrade by 50 so 350 gold free. Future Market charges 50 and you need to wait until 30 minutes into the game to equal those free boots. 40 minutes to catch up to the upgrade discount. The only time you want Future Market over Magical footwear is when you want to rush mobi boots to get your early roam going on.

I hope everyone finds these things helpful. Good luck on the rift!