private voting has already been changed to non-federated voting, so the original point is no longer relevant, but it absolutely is your problem if you want your votes to have any meaning. otherwise you could just not vote. if your instance is just telling me there are 20 accounts that vote very similar and I don't have means to verify authenticity of those accounts I'd rather ban them all and if you get caught in that then your votes don't matter anymore at all, at least as far as federation to my instance is concerned. otherwise I could just create a bunch of alts and downvote all your content, after all nobody could verify if this is coming from authentic users.

this basically boils down to requiring globally unique identifiers of users, and then still having them tied to voting, to ensure people can't just sign up on multiple instances to vote multiple times.

this isn't an issue with hard work, this is an issue with how much identifying information are you willing to collect and store, as well as requiring users to provide this. maybe you would be okay with providing your id on signup, but the majority of people won't be okay with that. this is also not data that i would want to store as a server operator, as this is a huge liability.

once you have this kind of unique identifier, and that allows you to prevent the same person from voting again, how do you imagine this could be done in a private fashion? it is just not possible. especially when considering your arguments about law enforcement requests, this would be even more transparent.