UnifiedPush is a protocol/library which allows users to choose how their push notifications are delivered, one of the options being FCM.
This would allow the F-Droid version of the bitwarden application to retain push notification functionality without the dependency on a non-free component (assuming the user sets up their own self-hosted push notification server), and the Google Play version can still function as normal thanks to the version of the UP library which includes a built in FCM ‘distributor’.