At best you have encrypted messaging, but this is not something that has been fully explored because many are happy to rely on tech like simplex.

The very next question after yours is "how do I revoke access to the content I permitted to someone previously?" And the answer is that you cannot. If you sent someone text or information that they are able to decrypt, then it is theirs forever.

Likewise, if you permit someone to see your birthday on Facebook and they take a screenshot, you can revoke their access, but you cannot prevent them from knowing your birthday or revisiting the screenshot.

Was thinking about this the other day from the perspective of running your own paid relay and certain (or all) notes would only be posted to that relay.

As was said already, once someone has access, to my knowledge there is nothing stopping them from sharing with others or rebroadcasting a note to their own relays.

But the initial distribution would be limited to only users who have access to your paid relay.