It could also contain non-public domain data, and you can't declare someone else's intellectual property as public domain just like that, otherwise a malicious actor could just train a model with a bunch of misappropriated data, get caught (intentionally or not) and then force all that data into public domain.
Laws are never simple.
It's not a setting that available to admins out of the box, and someone tweaking their instance not to process the request for deletion from the ActivityPub protocol can't really be detected from the outside, and sure won't be published.
Once it's out there in the fediverse, you can assume it's there to stay.