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).