TLDR: signal content in Apple notification can be retrieved even after signal app deletion.
I saw from this reddit thread: Signal messages retrieved from iPhone after uninstalling app. : signal
Referencing this news article: Pretti Killing May Affect ICE Prairieland “Antifa Cell” Terrorism Trial
The mention of signal is in court documents here: March 10: Federal Trial Day 12 - Support the Prairieland Defendants
Signal chat evidence from Sharp’s device (Exhibit 158):
Messages were recovered from Sharp’s phone through Apple’s internal notification storage — Signal had been removed, but incoming notifications were preserved in internal memory. Only incoming messages were captured (no outgoing).
I think on android, signal do not use Google’s push notification. They simple send a dummy push, and the signal app wakes up to retrive the latest message directly from signal server.
So Google never have your notification content. I am not sure if they do the same on iOS.
That being said if your attack model includes people reading your notification lock screen, then you should disable showing signal notification.
The message preview notification is handled similarly in IOS and Android. The issue isn’t people seeing the notification, it’s that the content of the message being passed to the phone’s launcher. Which is unencrypted.