the question of whether Mastodon should allow instance-only posts that do not go out to the Fediverse at large has been especially contentious. The final decision leaned toward discoverability
And this is why you have to assume your posts are actually public. The decision was made to amplify your voice.
@boyter There are instances already that restrict visibility that way. Once again, people should leave Gargron alone and get to work if they want something implemented.