This is how it used to work; notifications can be encrypted now and Signal uses an extension to decrypt them.
A Signal developer 12 days ago said Our FCM and APN notifications are empty and just tell the app to wake up, fetch encrypted messages, decrypt them, and then generate the notification ourselves locally.[1]
[1] https://news.ycombinator.com/item?id=47723445
alt Hacker News
A Signal developer 12 days ago said Our FCM and APN notifications are empty and just tell the app to wake up, fetch encrypted messages, decrypt them, and then generate the notification ourselves locally.[1]
[1] https://news.ycombinator.com/item?id=47723445