From Lemmy documentation:
We don’t delete anything from our database, just hide it from users. Deleted or removed Communities/Posts/Comments have a “restore” button.
But don’t take my word for it. Try it out yourself. You can make a comment, delete the comment (your username appears to remain) and then restore its content.
Reddit retains deleted user data, including after account deletion, and they absolutely sell it.
I dislike the retention of deleted data here, but it looks like account deletion at least is total, so it’s better than what reddit does.