I have no clue. My first thought would be to first study how mastodon behaves when someone removes a toot. IIRC there is some kind of propagation of the removal, depending on where it originates.
I don’t think it significantly changes things, provided removal also propagates under the same conditions as the copy. If content is removed from a forge, the copies that were made without human intervention should also be deleted. I did not really think this through though and it is likely more complicated. In any case, when there is federation there is a path for removal as opposed to when copies are made manually.
In the way fedeproxy is currently designed, the moderation is the responsibility of each individual forge. The only thing fedeproxy does is to ensure that every action is done on behalf of a user that exists on a given forge (their existence is verified) and not anonymously.