Damage creep and mages that can delete towers just as quickly if not faster all while bullying them with higher range, non existent manacosts, low cdr and mana refund mechanics have made them somewhat more redundant. ADC's do fabulous damage once they start racking up items, the problem is that so does everything else. Cdr is overabundant, so even if you do dodge some skillshots the next round will probably drop before you can even get out of range (mages tend to have higher effective range than ADCs' and have more than enough cc just lock them down and faceroll them, a potential I think should be reserved for low damage tanks with followup).
The damage creep itself ensures the assassins that don't suck balls and mages that have completed their first items can literally delete you often without even having to pop their ults. This means that peeling is less effective than ever since just 2 or 3 abilites is generally more than enough to erase a markmans health bar (think karma empowered Q shaving off 60%+ of an ADC's health bar and a Leblanc W as an example, it honestly doesn't take more than that and that's is very little).
Alas, marksmen have become more reliant than ever on supports and support items to keep them alive, which just adds to the extremes of the game. Support items wouldn't need to be as ridiculous if damage across the board wasn't so freakishly high and even then, if they are remotely behind it will generally not be enough, yet if they are ahead, it just makes the ADC and his/her support an unmovable wall that just facerolls you.
Damage creep has also made fighters have a field day picking of practically any ADC who isn't Caitlyn or Vayne because they need to be borderline broken to even work due to said damage creep (either they drop the marksman in a split second or get kited to death before they even get a chance).
Personally.
I wish we could just get rid of or revert keystone system entirely, it was meant to create diverse play but all it ended up doing was covering up weaknessess for already strong champs or further enchancing their strenghts to ridiculous levels, while leaving champions that can't really utilize them as effectively in the dust.
It has created a balancing nightmare ever since and the champion pool has not felt remotely balanced for one second since it's introduction.
ADC's always did and still do fabolous damage as earlier mentioned.
ADC's are still fully capable of bullying the fuck out of just about anything that isn't a mage in lane, it just doesn't matter much when they are so extremely easely deleted before they can dish their high DPS out in key situations.
Currently you either suck all the way as ADC and never manage to become effective or you somehow manage to get well farmed/fed because you play one of the few adc's with the required mobility/utility/safety to take advantage of or avoid the notorious 4man bot dive - thus making your way to draintank status while face-raping your opponents with endless lifesteal and super high dps.
Something really should give. Buffing marksmen's damage will hardly solve the situation, just make the game even more extreme since champions that can NOT engage them from range or easely dive them will become even more at their mercy, while those that can will still delete them with the exact same play pattern they do currently.