Fediverse

That's a pretty uncharitable interpretation, especially considering Lemmy is developed in and funded in part by the EU, and the "staying online forever" thing is a consequence of Federation (and one they're working on remedying).

If you were worried about this sort of thing, perhaps you should have done your research about the platform before making an account so you could bitch about it here. You definitely don't sound like the voice of reason when you couldn't be arsed to figure this out before you made an account.

It's been a problem for a while. Considering major social media companies have already gotten massive fines from the EU for violating the GDPR, maybe the lemmy devs will put more effort in setting up a deletion system once the EU sends them a fine for breaking the law?

Message your admin and ask for purging of that post/comment/user.

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).