Call it something like greentext or confessions or something. Anyone posting is automatically set to Anonymous with no link whatsoever to the original account for admin or users to track or in the logs/activity pub etc. Like the person will not get replies, notifications on their account for the post, or the ability to reply as Anon. Simply streamline creation of a throw away account using the existing credentials of an existing account for post access and to give automod a chance to act. Maybe bar new accounts or below a certain threshold of engagement.
I bet you could create a system that runs someone user and domain thru a hash to produce a different alphanumeric value. Display that value instead of user@domain.
So not exactly like 4chan, in that everyone has a user. But a username that is isolated from rest of fediverse. That would allow for ‘confessions’ type posts to not be linked to a standard account. And preserve ability of mods to ban the racists and trolls.
I’m not coder. But it seems possible.
Wasn’t there a 4chan themed instance?
EDIT: Probably this - https://usagi.reisen/