if you deliver a letter to your cousin, and they leak it to all their friends, is it the post system’s fault? instances federate by default, but private posts require actual intention. if i make a private post, explicitly mark it as private, deliver it to your instance and then your instance leaks it, i’d blame the instance, not the system. even signal can leak if you send your stuff to unintended parties.

someone can create a rogue instance

you shouldn’t send private stuff to unreliable parties. big software and big instances have a reputation, and it’s constantly up to you whether sending them something or not. when @sus@totally.legit follows you, check where they’re from. if you just accept follows left and right, are your followers-only posts really private? and if you direct message someone on some sketchy instance, you still need to trust them to respect your privacy. it’s the same on signal, e2ee doesn’t make a difference

this is why i completely blame pixelfed here: it breaks trust in transit and that’s unacceptable because it makes the system untrustworthy. you can get followed by sketchy people on mastodon.social and they will only see what you send them. in this case, other people can see what you post, regardless of you sending it to them or not, and regardless of the target leaking it or not