I don’t know where this myth came from, but your dummy have a right to delete your posts under GDPR. In the other hand, if you post a creative work, such as a poem - then it should be covered by copyright. See, for example, law.stackexchange.com/…/does-a-user-have-the-righ…

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).