Anecdotally, this has happened every time I've set up any kind of social media instance / discussion forum / BBS (back in the day!) / whatever. It immediately gets consumed by people who use it to host porn, and then all the intended users leave.

Have you considered creating a discussion platform where people can't post images, URLs / things that would be URLs if you added a URL scheme to them, ASCII-armored baseN-encoded anything, etc? For 99% of the discussion you want on the platform, text is all you need. For spammers and people who want to host porn, text alone is useless.

Plenty of spammers rely on text, though. A good chunk of my spam folder is text only.

Likely text containing URLs, though, which your email client likely helpfully auto-formats into links. Which is why spammers would bother. They need you to go visit something to make money.

There are types of bulk unsolicited email/web-comments which involve text without links, but they aren't spam per se; they're (the initial bait emails for) scams.

(And the usual solution to scam postings on forums, is to prevent people from sharing any off-forum contact details, except maybe via forum private message. No Nigerian prince is going to run their whole scam through the forum; they want/need to convert you ASAP into having a non-intermediated conversation with them. So they won't bother with any forum where they can't stuff their email address / Telegram handle / etc into each post.)

I've been itching for an experiment; might be fun to make a modernized BBS system for people to deploy.

If you're not spontaneously flooded with hentai, how do you even know your internet is working?

Japanese anime porn is not something that would trigger a banwave. A few posts from Trump, however...

Well, one of them is rotting the moral fabric of the country, and the other is just some hand drawn people having sex.