This is not true for Signal. Other apps may send the notification content but signal uses FCM to push a simple notification to wake the device and tell signal to fetch the actual notification. You can use the full text / info notification and know that Google does not see it.
https://discuss.grapheneos.org/d/1279-sandboxed-google-play-for-push-notifications-breaks-privacy/9
We are both right 😆
It is true for Signal on Apple devices.
It is not true for Signal on Android devices*
*Well I’m using grapheneOS so I feel more comfortable in my case but a regular Android device with full access Google Play Services? That I’m not so sure about. It’s conceivable that Google has a way to read the final notification (FCM push -> Signal fetches and displays message -> Google can read all notifications on the device, FCM or otherwise) 😬