There's a form of deniable authentication where you have the ability to forge the signatures of messages once you receive them, so you could alter them on your device.
It's irrelevant, though, because no chat app actually has this feature, so no criminal does it. And the cops don't go by the encrypted bits, anyway - they go by what it says on your screen, or their screen. (Which does mean that if you hack into your own app's database and forge messages, you can convincingly incriminate people, yes) Research into deniability protocols is cool research, but with no relevance to the real world of cops vs robbers.
It's irrelevant, though, because no chat app actually has this feature, so no criminal does it. And the cops don't go by the encrypted bits, anyway - they go by what it says on your screen, or their screen. (Which does mean that if you hack into your own app's database and forge messages, you can convincingly incriminate people, yes) Research into deniability protocols is cool research, but with no relevance to the real world of cops vs robbers.