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.