I know this is gonna offend all the mage support players and whatnots, but this is coming out of an (ex) adc main's perspective.
Mage supports are the worst. Playing with and against them. So let's say you play against a mage support: there goes your laning phase. You wanna last hit a minion? 50 spells in your face, with insane damage that can easily kill you. And other than a blitz or thresh hook, you can't just hide behind a minion in order to dodge a brand w or a lux e or pretty much any mage's spells. So you either go full sustain runes, which fall off insanely hard, or you just get fucked in lane.
And to those of you thinking "Just take a sustain support, like soraka"; here comes the second point into play. Most supports are fucking idiots, honestly. They will pick a useless mage support into anything.
The enemy has a zed or rengar that can one shot our jhin effortlessly? Let's pick brand and just watch our adc die repeatedly and notice that we can't ever save him! They are useless against any engage or poke or whatever, they only deal dmg minus the "supporting" part. playing with a mage support is cancer. They lack the cc and/or tankiness to help let alone save you in any given scenario. And anyone who thinks they can, should play a single game with a mage support against an engage support or any assassin reallly.
So obviously mage supports are cancer to play with or against, so just remove them. Remove spellthief's or just permanently suspend anyone who tries to pick brand support with jhin against rengar, tristana and rakan.