> Even better, flipping "public/private" has absolutely no need to delete any information at all.

Of course it does. You can’t watch/star a private repo unless you have access, so all watchers/starrers should be removed. If you kept the data as-is watchers would still see the repo updates in their feed, which is the opposite of the 'private' feature promise.

Sure, if you don't change any part of the system it has to be that way. But there's no reason a private switch can't just suspend stars/watchers instead of permanently deleting them.