A friend made a keyboard for iOS because he didn't like any of the existing ones, and it doesn't send any data anywhere: https://typeneuma.com
Futo keyboard on Android has a transformer LM option that uses local ML for text prediction, I've found it to work very well. As far as I'm aware it's completely on device.
“Generated entirely on device” seems like an omission that doesn’t cover diagnostic, telemetry, “product improvement” logs.
It can be the same way as it works now, hash table with hits stats.