Hey long Riven otp here,
Rivens winrate is as it is because of her playerbase and her playerbase is OTPs only. Since OTPs know how to play her and know how to play around their matchups, they can pull of a possitive winrate, moreover unlike other OTPs Riven players don't have less than 500 games with her, on one account at least. Usually Riven players got 1000+ games, those that have less are starting to grasp her and because of that they can pull of 'again' a possitive winrate.
Unlike Zed,Yasuo etc. Riven is ONLY picked by OTPs. We all picked every champion in ranked when we want to mix things up but we know we can win with it, since those champions aren't that complicated. We all picked Zed even though we don't play him regularly, Yasuo, or any other you mention. You can think of any champion in the game which you would pick when u want to just switch things up, but you would never think of Riven to do with. Do you see any toplaner playing Riven? No you see Irelia, Camilee, Kled, Garen, Darius, Urgot, Aatrox. These champions aren't complicated but deal good damage, scale excellent and are meta. Do you pick Riven when you are autofilled? No, you don't.
So this is kind of ignorant to say without thinking things through. Riven is an OTP champion only with huge playerbase, nobody picks Riven and says 'imma try her out in soloQ' , and even if you did by some dumb miracle i bet on my life you went 0/10 or more. Screenshot it and send it here if you did, if you aren't a Riven player. Riven is the only champion in the game you can't just pick for a heck of it.
Edit: Those that say Riven is OP are either lower elo so their enemies don't know how to deal with Riven or are ignorant. The higher you go, the harder it is to do something with Riven, people know her and her weaknesses to some degree. Did it ever occur to you that players higher than Platinum don't complain about her? I have never seen a Challenger,Master,Diamond player complaining about her. Usually it's a Silver,Bronze,Gold player. Platinum players are on the edge. Thing about this, if you are complaining about her you are not higher than Plat. Therefore i wish those that are complaining about her to send me their opggs so i can prove my point.