First off, to clarify, I don't mean Riot only buffing/nerfing champs with low/high winrates. What I mean is people on the boards basically implying that a champion is balanced or fair or what have you, simply because their winrate is low.
Now don't get me wrong, when a champ has a really high winrate, there is definitely an issue there. But there are and have been champs with below 50% winrate that have been nerfed hard and/or repeatedly, because they are, and this probably isn't the best way to describe it, unfun or frustrating to play against. It's why LB got nerfed back in Season 3 and had her silence removed in Season 4. It's why Azir has been systematically nerfed almost from release. It's why Zed is currently up for nerfs. It's why certain champs either have been reworked or are on the list for one.
To reiterate, there are more factors when it comes to balance than just winrate.