These are the first examples that come to mind. There are many more with similar userbases that has migrated from them. Don't forget all the Pizzagate/QAnon stuff, incels, etc. Here's a longer list: https://en.wikipedia.org/wiki/Controversial_Reddit_communiti...
Also, quarantined != banned. It's still on reddit, and those users spread out onto other non-quarantined subreddits and spew the same kind of garbage.
The problem with having a cesspool is that it attracts lots of those types of people to your site in the first place, and ingrains it in them that that kind of behavior is acceptable. They then start acting like that elsewhere on the site and lower the level of discourse everywhere.
Ban the cesspools and most of those people will actually leave the site for good, as what drew them to it in the first place is now gone. You also no longer have any cesspools serving as negative guideposts for making bad contributions. So if you moderate your site well, and just get rid of this kind of stuff, it ends up much better in the long run.
Also, quarantined != banned. It's still on reddit, and those users spread out onto other non-quarantined subreddits and spew the same kind of garbage.