Yes, and my point is, that the person running an instance has to comply with the gdpr if they are within the EU.
It doesn’t matter if data has already been propagated somewhere else. On that instance, data needs to be able to be fully deleted. For the matter of deletion, it is irrelevant where the data might have been pushed or mirrrored to, that is a seperate issue, which still needs to be dealt with. But one cannot argue that deleting is pointless or needn’t be implemented, just because “public” data is already mirrored elsewhere. The people running “elsewhere” have their own compliance to deal with.
There already is federation of deletion. It’s not even something that needs to be implemented.
I have less of a defeatist attitude about privacy. Same way I don’t think absitence is the only true way of contraconception. Privacy, yes, even if public spaces is possible. It’s not easy, it won’t just happen, but it is achievable. Needs a lot of work from a lot of people, but it is doable.
I don’t expect you to change your mind on that.