I glanced at the Phoenix source code and didn't see a way to disable the warning. There are three warnings bundled together: Tor, FCM, and power saving. https://github.com/ACINQ/phoenix/blob/e73fb0e5aba1e890d141e6433d810ad7ab1f8047/phoenix-android/src/main/kotlin/fr/acinq/phoenix/android/home/HomeTopAndBottom.kt#L233
However, there are discussions about alternatives to FCM: