I don’t know where this myth came from, but you don’t have a right to erase your public posts from there internet under GDPR. See, for example, https://law.stackexchange.com/questions/32361/does-a-user-have-the-right-to-request-their-forum-posts-deleted
If anything, you might have such rights under copyright law, if your posts cover the threshold for copyright. In that case, you can ask server admins to delete them, and they will have to comply. But the request has to reach them (if they’re defederated, the delete button won’t teach them, and you’ll have to contact them separately).
Stop asking for pseuso-privacy features. The Fediverse is public by nature. Any “measures” to control access to the public posts on it are just lying to users.
Server owners should be able to control who can access their servers - but that is NOT - and should NOT be - treated as a privacy feature.