Then do it insecurely and warn the user. 'We can't figure out how to do this securely' is their BS excuse for every bad design decision or unimplemented feature.
You might say 'but they don't want to make people less secure, people will get the wrong idea!' But they do this already, in ways that are much worse than allowing the user to make a security decision for themselves.
You can change a setting to prevent screenshots inside the Signal phone app, so you can't take screenshots. Your conversations are now secure, right? Nobody can take pictures of your disappearing messages! WRONG. You can turn on that feature and I can still take screenshots all day, including disappearing messages that you send to me.
Likewise, Signal can't tell if you're downloading pictures or copying text I send to you. You could be backing up everything - my only 'assurance' is that you probably aren't doing it because it's inconvenient.
You can change disappearing messages timer to anything you want! Great! But the change of timer is itself a message. So if we are arrested and police get into one person's Signal, they can see when disappearing messages were turned on and when the timer was lengthened or shortened. Sure, the messages disappeared, but what were you doing on August 23rd at 7:39pm that made you change the timer to 10 minute4s for 3 hours? We know where you were because of your phone's IMEI, I guess we will tell the court that you were trying to cover something up during those 3 hours and charge you with obstruction of justice.
I have asked them to change the latter behavior repeatedly, explaining why it could be a problem for users, and all I ever hear is 'good point, we'll look into it' even though there's no reason that information should be stored.