[notpushkin]

I think Mastodon has a pretty good balance here – when you try to send a DM it explicitly tells you that it will not be encrypted: https://u.ale.sh/Vo1ahx.png

And the linked privacy policy goes into further detail (at least on my instance, mstdn.io):

> Please keep in mind that the _operators of the server and any receiving server may view such messages_, and that recipients may screenshot, copy or otherwise re-share them. Do not share any sensitive information over Mastodon.

Overall, I think it's safe for most chit-chat, and for anything more serious you can add link to Matrix or your email and PGP key in your profile.

As a sidenote, I'd also like to point out that a lot of serious communication nowadays still happens over unencrypted email. You can consider it whataboutism, but it's still worth remembering IMO. (And of course, like others pointed out, DMs on Twitter aren't encrypted, too, so it's the status quo here.)