Through #ActivityPub, #Mastodon privacy settings rely on voluntary cooperation. You request that instances only share your content with the audience you specify, but there is no real way to enforce that.
This comes as a surprise to many users.
Me, I think I’d change the UI to call it “suggested broadcast”’ rather than anything related to privacy so that users are more aware of where their content might end up.
@volkris The only thing I can think of to mitigate this problem is some sort of E2EE like Matrix, although that is far more complex.