If you enable follow requests, and trust the admins of your followers, does that work as a privacy feature? From what I know, such posts do not get federated to unrelated servers, right?
Anyone thinking their post is only seen by their followers when they have federation turned on is grossly misinformed.
I think this is simply a social problem — when sending a post to your followers, you have to trust that they do not share your post. The same applies in private messaging. You have to trust the recipients.
I don't agree in this case where the system is designed to spread those posts. Fair enough if you don't want that to happen, in which case don't use a federated system where that is a design goal.