mastodons biggest shortcoming remains the lack of e2e encrypted DM's. Giving volunteer internet janitors access to your private messages is a recipe for disaster
You can still high-horse Mastodon: just tell them they shouldn't have private messaging at all. That seems like a reasonable take.
Could you clarify whom you hope that a message should be able to be read by?
- the intended recipient(s)
- parties involved in the intermediate storage system
- middleboxes / parties in the transmission path
- unintended recipients that the intended recipients forwarded the message to accidentally
- unintended recipients that the intended recipients forwarded the message to maliciously
1 is obvious and I think it is bad faith that you are asking.