Nice to see Signal (not the default Messages app) in the Smart Reply demo.
I use Signal because I don't want Google reading my messages.
I don't want Google reading my Signal messages on their devices or mine.
They show up on all apps that provide messaging.
(The app of course also has a choice of encrypting the payload - it's free form after all.)
push notifications services can merely send instructions, and the visual notification can be an object formed completely client side.