"Bridgefy is a developer-friendly SDK that can be easily integrated into Android and iOS mobile apps to make them work without Internet. Companies increase engagement by letting people use their apps when they previously couldn't."
This is becoming rather popular in Hong Kong, lately. Ad-hoc mesh network over Bluetooth. Direct messages are encrypted, but Bridgefy also has a broadcast feature which is unencrypted and open audience.
The Bridgefy messaging app for iOS and Android can be found here: