In the decade prior to me abandoning cell phones entirely I used E2EE email frequently on Android via OpenKeychain and K-9Mail with a Yubikey.

I did not want to be locked into Google or Apple ecosystems and the ship on a truly FOSS Android device has long sailed, so I ported my number to a VoIP provider and only use PCs now.

To your point -any- E2EE on proprietary platforms like iOS or Android cannot be trusted for high risk comms, so at my infosec job we exclusively use Matrix from QubesOS workstations.

But yes email is normally the go-to fallback except with Gen Z who almost universally refuse to use it having grown up exclusively using corpotech.