Signal on Android is doing just that and I love it. But they announced that they will remove that feature due to too many users thinking that an SMS sent with Signal is secure (despite an open lock displayed next to the message)
I know this is off topic, but having spent a significant amount of time convincing people around me to use signal, once they remove this feature, literally 100% of the non-tech folks I've gotten onto signal will drop it and move back to SMS. It makes me so sad.
