Caitlyn is a textbook case of why you shouldn't listen to the front page of gameplay for balance.
Coming out of the "adc in 2017" period, Cait was doing as Cait always has.
Bullying in lane, falling off in the mid game, and scaling back into one of the better carries in the late game.
Was she strong? Sure, but not to an unreasonable degree. She was a popular champion with a low skill floor and a mid-high skill ceiling, so a high winrate makes a lot of sense.
Despite the weaknesses and counterplay of the champion being obvious, the boards screamed for months that Cait was utter cancer with no counterplay.
As a result, Cait got hit with a flurry of nerfs in season 7.
Not counting nerfs that were later reverted...
- Removed hurricane stacking headshot.
- Reduced passive attack speed.
- Trap headshot AD ratio cut from 70% total AD to 40/55/70/85/100% bonus AD.
- Trap reveal cut from 8 seconds to 3.
- Trap duration cut from 90 seconds to 30/35/40/45/50.
- Base HP reduced by 50.
- HP regen reduced.
As a result, Cait no longer has a strong early game, while her mid-late game is pretty much unchanged.
A sub-par early game and an utterly shit mid game, all building up to being sub-par again in the late game.
Despite being the type of champion that has a higher winrate when balanced, she's sitting at a 47.75% win rate in plat+ according to champion.gg, and a 43.6% winrate in diamond+ according to League of Graphs.
Late game beasts like Vayne, Twitch, and Jinx are all rocking a 53.8-54.5% winrate against Cait because she's not strong enough to stop them early anymore, and can't compete with them in the mid-late game.
The only ADC's that are so bad that even Cait has a positive win rate against them are Lucian (who got shat on in preseason) and Cait herself (Don't ask me how this happened, ask whoever made champion.gg).
Tl;dr the boards bitched about Cait enough that she got gutted, despite not being that strong to begin with. She's now incapable of doing her damn job.