Anyone assuming anything on the fediverse is anything but public is wrong and hasn't spent any time thinking about what the fediverse is. That may well be a problem that needs to be addressed, but the fundamental design of the protocol means, at the very least, server admins can see everything the users on their server do. This is a problem on any system that does not use end to end encryption.
Realistically there is only 'public' and 'I didn't press send'.