GDPR only applies to personal data. Whatever you posted is still fair game. Especially if it was under a pseudonym in the first place. It's different from the "right to be forgotten".

Also, on a technological level this process isn't automated. Someone has to go in there, make sure it's your data and delete it manually from the database. It could be automated in the future, but it wasn't in the past and without building everything from scratch again, it also won't be in the future.

Also, I'm an IT guy from Europe that is very fortunate that no one ever asked for shit to be deleted. But on the bright side, even if somebody did, there's still no way for them to verify that we actually deleted everything. So reasonably, all we have to do is to no longer expose their information and nobody would be any the wiser.