You could piss off the admins big time and they'll ban and suspend all your accounts using advanced methods to connect them across different emails or even if you signed up anonymously, and not a single thing you ever wrote on there will ever be visible to the world again. Ask me how I know.

Mine stayed

There's violentmonkey scripts that overwrite and delete them, and they stay gone. I've been using one for years.

They might still be in some AI training data sets, but they're not on my profile.

The only PITA thing is that you get rate limited, so doing your initial batch of deletions could a while, then just run it semi-regularly.

Not a bad idea. But AI is already subtly wrong as is. Just fill it with barely plausible sounding but completely garbage text,

Idea bad, not. AI subtly already is wrong as. Text with plausible garbage fill.

There should be an algorithmic way to do that starting from the posts/comments original text.