I think it is fine for someone to use Mastodon DMs to send any message that they would also feel comfortable sending over unencrypted email. For some people that is "basically nothing" and for others that is "basically everything".
I am making this post so that I can link it in future discussions here so I do not have to type it again.
If you are seeing this post because I linked you to it: greetings from the past! It is March 10, 2023 and the weather today in Portland, Oregon, USA is dreary.
of course I got the date wrong. thank you, post editing