I think the legal defense would be whether the site acted reasonably and liability would be difficult to establish without harm. If people use your site to coordinate doxxing someone and you don’t take reasonable actions to prevent, stop or legally identify the users behind it, then you’re going to have a tough time getting out of liability. AFAIK doxxing isn’t illegal but harassment can be.
Side note: I wonder what the new requirements that beneficial owners of legal entities be disclosed will do to 4chan, 8chan, etc.
If there is harm, then without section 230 "acting reasonably" is not a legal defence - you (as the website/business/service allowing user-generated content) will be targeted by lawsuits even if you act reasonably simply because may be easier to extract monetary compensation from you than from a random teenager who actually posted the harassing content. Even if you identify that poster and ban them, that does not undo the harm and liability, so you'd be the target of expensive lawsuits. You would likely have grounds for a counterclaim to that user to compensate you for the losses, but that won't help you much, as your costs would be the total claim of the harassment + lawsuit cost, and your compensation would be limited to what you can recover before the user files bankruptcy.
There's no grounds for them to sue a platform because of the immunity granted by Section 230 which was passed 16 years before the events of Sandy Hook as part of Communications Decency Act of 1996. The above comment was about a hypothetical situation if that immunity was repealed - then we absolutely should expect every platform to be sued into ground unless (and even if) they censor everything.
Side note: I wonder what the new requirements that beneficial owners of legal entities be disclosed will do to 4chan, 8chan, etc.