Almost _all_ my Signal chats are on 1 week or 1 day disappearing settings. It helps to remind everyone to grab useful info out of the chat (for example, stick dinner plan times/dates/locations into a calendar) rather than hoping everybody on the chat remembers to delete messages intended to be ephemeral.
The "$person set disappearing messages to 5 minutes" has become shorthand for "juicy tidbit that's not to be repeated" amongst quite a few of my circl3es of friends. Even in face to face discussion, someone will occasionally say something like "bigiain has set disappearing messages to five minutes" as a joke/gag way of saying what used to be expressed as "Don't tell anyone, but..."
(I just looked it up, https://signal.org/blog/disappearing-messages/ from Oct 2016.)
I think encrypted messengers should have a "completely off the record" mode that can easily be switched on and off. Such a mode would guarantee that your messages are never stored anywhere that might become permanent. When you switch it off then everything is wiped from memory. That might be a good time to ensure any keys associated with a forward secrecy scheme are wiped as well.
The analog hole ALWAYS exists. Pretending it doesnt is ridiculous.
Not if the message has already been deleted. Auto-deleting messages are so the recipient doesn't have to delete them manually, not so the recipient can't possibly keep a copy.