Conspiracies using Signal and similar services often fall apart because someone in the conversation is undercover or has a change of heart: someone thought he was signing up to a protest movement and then finds out that others want to kidnap a politician and hold her hostage (the governor of the state of Michigan, where this happened) and sends the messages to the cops, even though they were set to disappear.

Figuring out a solution for this would enable new kinds of horror, like when in 1984 Winston is shown a photo that implicates the party leader in a crime and his torturer then tosses it in the "memory hole" to burn it up. You can see evidence of something horrific but you cannot share it, cannot prove it and then it disappears.