I wonder if that's true for mobile versions too. Android notifications will bypass the lock screen to show the incoming messages from other messaging apps. I also wonder about the app manager that shows screenshots of apps in use. They appear not to show entered text but that doesn't mean screenshots WITH entered text were not also captured.
Signal messages aren't disappearing because of a bug, report says https://www.cnet.com/news/signal-messages-arent-disappearing-because-of-a-bug-report/
