As a general note, the design of Lemmy is around preserving information against censorship over having private conversations (like Matrix for example). You're probably aware that once data is delivered to other servers, the origin server can't guarantee deletions of data from them.

I'm not an admin, but I remember that Lemmy admins are able to purge content from their server, and send out a request for other servers to purge their copy of data which by default would be accepted if received from the canonical server. This was used to remove some harmful content that was posted on Lemmy a few times. This feature isn't user accessible, at least as of now.

Media uploaded to Lemmy by the user can be deleted.

Edits are propagated through federation, so if you are concerned, you can edit posts you want to delete before deleting them which is close enough.