[Macha]

Legally it's governed by the terms of use which for most sites say something along the effect of you retain copyright ownership but transfer a non-revocable unlimited license to the platform.

Sometimes other legislation, like EU right to to be forgotten, GDPR, etc. overrides that and they have to take it down, but otherwise they could pretty much do whatever.

[suddenclarity]

GDPR doesn't force a forum to take down content unless it contains private information. It's enough to just change the username to a random string and then it's no longer tied to you. Authorities in my country published guidelines about it because plenty of people saw it as an opportunity to get rid of shit they wrote when they were younger.