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).