Good luck with that. Once the post federates out, the host instance can request for deletion, but any federated instances that receives the content doesn’t necessarily have to follow that request. They could easily modify their instance to not delete, they may reactivate the content from moderation log, they might have backup strategies that involves retaining data (for their own local legal reasons), etc etc.
It’s probably best to assume any content that you post on Lemmy are out of your control and will live for much longer than you’d expect.
This is not limited to just Lemmy but any federated systems. So regardless centralized corporation behind the service, or an open federated system; one way or another, whatever you post out there, its no longer yours to control.
I don’t care for the argument one way or another; I’m not an EU resident and the whole thing is irrelevant to me as an individual.
I’m merely pointing out neither the Fediverse/Lemmy/etc. nor Reddit as a platform cares for EU’s privacy concerns, and people should be well informed when entering either platforms, so they’re not doing so with the false sense of security that they’d be able to exercise those government granted rights effectively.