After sending a DM, the profile lacks access to it. I can see my posts and public comments, but not my DMs. Thus there is also no way to read or edit DMs Lemmy users have sent.
update
As @[email protected] points out, sent messages are accessible in the ALL tab. Once my DMs are rendered, indeed there is an option to edit them just like a public message. But presumably due to another bug, Lemmy recipients are not likely notified of edits (untested).
Irrelevant.
It’s unclear why you talking about non-federated systems. But in the design of any comms system there are multiple ways to support edits:
W.r.t email, MS Outlook indeed has a mechanism to retract a message if the recipient chooses to enable it, but it only works on unread messages.
BTW, my bug report indeed is not a bug, but not for the reason you mention. It’s not a bug because it was a user error (I forgot how to reach sent messages). Lemmy does in fact support editing messages (DM or public just the same).