Reddit has informed moderators of communities that are still private in protest that they will lose their mod status by the end of the week. Thousands of communities went dark earlier this month to push back on the company’s planned API pricing changes.
When you delete a post, your home instance sends a federated deletion command. If every server is running lemmy/kbin as-is, then your post will be completely gone. But anyone could modify their instance to not delete posts.
I think if you want GDPR style deletion, you would have to contact each instance admin individually.