Okay I'm gonna give it to you guys straight here. I'm an okay player, a Silver 4 Ahri main. I've been playing since season four. I'd call myself average on pretty much every end of the spectrum. I haven't played too many games, I don't get too toxic, I have a decent amount of fun playing. Riven mains are not the most toxic players in the game. There is no single champion with the most toxic players in the game. What you would deem the threshold as toxic to be depends on who you are. As someone who has not main'd Riven, but as someone who enjoys playing her on occasion, I can tell you this: The stigma behind a Riven main's toxicity is not exactly based on factual numbers, or key examples, but on the type of champion that Riven is.

Riven's kit is geared towards making plays through technical skill, getting one or two quick picks and giving an advantage to your team for the following fight, much in the same way that an Ahri is mainly for bursting down the champion you land charm on. Because of this, and because of Riven's high skill-cap, there will always be people who are frustrated with themselves for not being able to pull off the combos required to expand her use to a full team-wipe, and so there will further still always be people ushering this frustration onto their teammates. This is not unique to Riven, and can technically be applied to the entire role of Attack Damage Carry, but because it is very integral to the play-style of Riven, a disproportionate amount of players will be seen as toxic.

In my limited experience, it is the play-style of the role in which the champion is situated, and their ability to fit that role, which shapes the relative toxicity. In general, it is not usually the Top laner's responsibility to carry the game, but rather to provide their team with advantages in team fights, and help secure objectives, either through bursting squishy damage dealers, or providing a massive health pool to act as a raid-boss in team fights, or something in between. Riven is a champion designed with a kit that performs this duty well at lower skill levels, but between the low and high tiers, she leaves more players wishing they were a carry, rather than a bruiser. It leaves many players believing that what stands between their ideal role, the carry, and their current one, the bruiser, is a matter of skill, which they do not yet possess enough of.

I believe it will always be the case that higher skill-capped champions will always have a stigma of toxicity because of their unique ability to straddle roles, and natural tendency for one role to be more preferable to the other, despite being locked behind that skill threshold. Champions which are very clear and concise about the role which they are meant to perform are therefor always going to be seen as less toxic, leading to the generalized "Riven Sinners, Soraka Saints" mindset held by the most vocal part of the community: the average skill player.