CSAM and piracy aren't the worst of examples on a site like 4chan. People... like clinically insane people just keeps scriptbombing someone's private addresses and bomb threats and other schizophrenic copypastas(not regular ones like Blackbird speed checks or "Glasses are versatile", but more like cognitohazards) as well as there will be viagra/cialis/poker/prostitution/roblox spams. I think those in that order should be the primary concern for keeping BBS running because those chase normal people away.

For the two categories mentioned above that are plainly illegal, I think, ironically, you can probably just keep AI classifiers running and wait for calls from NCMEC or DMCA emails, exploiting safe harbor clauses and/or by pointing at reasonable measures you've taken. If you have a large post volumes and short public log retention(private logs will probably have to be kept), that would help keeping the pressure low for those.

Deciding what is spam and blocking it is moderation or "censorship".

If 4chan didnt have spam filters, the users would be begging for it almost immediately because the website would become unusable.