I currently use Telegram for my friends and family, but have reluctantly come to the conclusion that the UK Government is either reaching agreement for backdoors with messaging services, or is trying its hardest to.
I’m also on Element/Matrix. Before I try to get my contacts to join me on there, should I be aware of any privacy issues or is that a good place to head?
That’s simply false. Signal Notifications never include the content of the message or any metadata, no matter if they’re sent over FCM, APN, WebSockets or UnifiedPush (via mollysocket). That wouldn’t even be possible, since the Signal server sending out the notification doesn’t even have the key to decrypt the message. Only the users involved in the conversation have the keys, that’s how end-to-end encryption works. Signal simply sends an empty message via FCM (or any other push system), and the Signal app on your device then receives and decrypts the encrypted message and shows you a preview of the message content as a notification on your operating system.
And every build of the Signal client for WhatsApp also supports WebSockets as a fallback push notification system, in case Play services aren’t installed or can’t be reached. The only reason why FCM is used by default is that it saves some battery, because it only maintains one background network connection for all apps, instead of each app handling notifications themselves.