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).
Good? No.
I think it is telling about Signal, though, that despite being in a privacy-unfriendly jurisdiction, federal authorities can only extract data from it when its users mess up.
I don’t think you’ll get much better until some of these other services mature more. Some of them seem painted into a corner where improving them further seems to involve rewriting big sections of them, like Matrix, so I am less optimistic about those.