But they’re not.

According to the actual GDPR website, data tied to a pseudonym is only sometimes personal information if it’s easy to tie it to a real identity. Lemmy posts don’t qualify.

It’s not possible.

By design, everything you posted is shared to hundreds of other servers, all of which are capable of doing anything they want with it. I can guarantee you that there are several that are archiving anything and everything that gets federated to them and will not remove that content when the original server does.