Is there a term for any application which offers full control of your messages then, ie, I send you messages on Signal, but I can make them self destruct and you cannot screenshot them? (Pretty sure Signal allows this?). Nothing stopping a user from taking photos of the screen using another device, of course. Or running their own fork of Signal (which, when run from the open source for Android at least, runs on production).
Taking photos of your phone screen is the main loophole and is completely undetectable. Exactly what happened to Waltz and what caused TFA.
If you really need to, you can combine this with a rig that holds the phone and the camera just right, controls the lighting, and interacts with the phone via a hotdog mounted on a gantry. Come to think of it, any 3D printer can be adapted to archive Signal/Snapchat/etc. messages in a completely undetectable way. Could even reply if you rig up another phone to talk to your hot dog finger + camera robot.
Dunno. Like I said, there's no way to do this effectively without some form of leverage over the counter-party. This sort of thing is why SCIF's exist, and is an example of the more extreme ends of leverage, but it still ultimately comes down to leverage: they can make you delete the message and will throw you in jail if you figure out a way to evade it.
