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).


As I already replied om one of your other comments:
Is is 100% because of firebase. Here is an example payload from firebases official document:
{ "message":{ "token":"bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...", "notification":{ "title":"Portugal vs. Denmark", "body":"great match!" } } }https://firebase.google.com/docs/cloud-messaging/customize-messages/set-message-type
Notification history is purely local to the device. It is not sent to any servers.
my reply on another thread: https://sh.itjust.works/comment/24730726