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FBI Extracts Suspect’s Deleted Signal Messages Saved in iPhone Notification Database
(www.404media.co)
This is a most excellent place for technology news and articles.
I imagine a similar exploit will work on Android devices, as well. Wouldn't have considered it, but it may be a good idea to figure out how to disable the content from appearing in the Android notifs, too.
It's not an exploit. It's a built-in setting in Signal, and the Android options are identical to the one displayed above. You can turn off notification history in Android as well, so it has no stored record of cleared notifications at all.