what's funny is that Mastodon is wide open, like email, and direct messages can easily be read on the server.
however.... ActivityPub (the base of mastodon) is soon coming out with Messaging Layer Security (MLS) which is an end to end encryption.
AND Mastodon is run on multiple servers. (I actually host my own for the family on a crappy old server in my office)
so it's quite possible that the safest and most stable message system, that cannot be taken down or corrupted by the government, may actually end up being activitypub. lol