I don’t know too much about the GDPR, but that’s helpful to know they do regulation on sites even if they only have one or a few E.U. members.
My understanding is the primary purpose of that is for the sake of protecting individual privacy, so would be in favor of allowing people to delete their own accounts or for user requests for that to be approved. I know of one discourse forum that doesn’t offer that service to users, which does include some E.U. residents, so seems that isn’t a requirement of the GDPR that people are always able to have their accounts deleted.
The main reason why it’s not necessarily always a great idea to allow people to delete an account is that, if a forum has any purpose besides being a place for idle chit-chat, there needs to be accountability with what people are posting.
Specifically for if a forum is open to the public anyone who qualifies for a library card can set up an account and maybe post something that should probably be reviewed by and responded to by law enforcement in their jurisdiction.