It's an interesting edge case. It's pretty clearly completely legal in the USA, though the content is grossly offensive. Some of the individual posts are almost certainly defamatory, but §230 shields the site itself from that liability (though not the posters).
This brings us back to the real question: should extreme unpopularity alone be enough for deplatforming in a free society? Or should actual laws need to be broken?
I think it's much simpler: make sure the content isn't legal to begin with. doxxing seems to broach on 4th amendment rights and we're no longer in a day and age where we will freely give out our address in newspapers. I don't know the fine legal details, but make a DMCA equivalent of private information. If you request a takedown of your identity and they don't comply, throw the book at them.
Likewise, death threats and that kind of harassment isn't protected by 1st amendment rights. I don't think sites should be sued by consumers over user submitted posts, but they should be held accountable for at the bare minimum removing that harassment in a timely matter when asked. If it doesn't happen often enough, fine them.
This brings us back to the real question: should extreme unpopularity alone be enough for deplatforming in a free society? Or should actual laws need to be broken?