I struggle to think of examples, and the worst ones are things that might be described as "sounds kinda racist but the person who wrote it is probably not the cross-burning type".
Sure, I could find it if I went looking. But I don't.
You could argue that this is because of some kind of benevolent filtering, and I'll grant that dang (of HN) does a great job. But I think it's mostly because I don't delve into the kinds of places where that would happen.
So I'm honestly just not sure what you mean when something will devolve into 8chan. I've heard of that but I don't know anyone on there, so I don't even really know what it is or how the devolution might happen. Mechanically, what would take place? Would I follow a friend, and then start getting weird flat-Earth stuff because of their second cousin?