ActivityPub, the protocol that powers the fediverse (including Mastodon – same caveats as the first two times, will be used interchangeably, deal with it) is not private. It is not even semi-private. It is a completely public medium and absolutely nothing posted on it, including direct messages, can be seen as even remotely secure. Worse, anything you post on Mastodon is, once sent, for all intents and purposes completely irrevocable. To function, the network relies upon the good faith participation of thousands of independently owned and operated servers, but a bad actor simply has to behave not in good faith and there is absolutely no mechanism to stop them or to get around this. Worse, whatever legal protections are in place around personal data are either non-applicable or would be stunningly hard to enforce.
@Deceptichum
Yes. DMs on Twitter are Direct Messages and are supposed to be private messages send to someone else that no one else can see (except server admins, et al, as we are talking about here). If you send a DM to someone on Twitter or whatever social media (they use DMs to mean private messages on Instagram as well) it’s not on the public feed, no one can search it. Like having a text message conversation