Yeah I understand that of course a random instance could just change the code so that nothing is deleted. However, what I meant is that in the documentation states that the instance can use this Tombstone instead of deleting and it seems like it is completely "fine" and within the rules to do so. I am referring to this:

... the server receiving the delete activity SHOULD remove its representation of the object with the same id, and MAY replace that representation with a Tombstone object.

From https://www.w3.org/TR/activitypub/#delete-activity-inbox

So they should do it but it is also OK to use this different thing. Or am I misunderstanding this comment and it means that instances need to delete the object, and after the fact it is allowed for instances to furthermore make this Tombstone to somehow track that yes there was a deleted object here?