We all know Signal, Matrix, Telegram, SimpleX, etc… But if you can’t access the internet you can’t communicate. Pretty logic. But would it be possible, at least theoretically, to create an app that permits to message people even if the internet goes down?
It might be a dumb question I really have no idea to be honest.
Depending on how far you’re willing to push the definition of “messaging” you could look into getting your ham radio license. It can’t possibly be censored and allows you to communicate all over the world. You can even build your own radios if a government cracks down on them for some reason.