[upofadown]

If the user can get immediate access to older messages then normally those messages will be available on a confiscated phone. That's why things like Signal have you set a retention period. A retention period of zero (message is gone when it scrolls off the screen) is safest.

If you want to protect older messages you can have the user enter a passphrase when they are in a physically safe situation. But that is only really practical for media like email. Good for organizing the protest but perhaps not so great at the protest.