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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.
Notification logging is usually done by some other part of android as far as I know. GMS is the typical way to deliver notifications and is a far more serious privacy concern, since it also directly passes googles servers and is not encrypted. However as others mentioned, signal does not send contents there, message notifications with the message contents stay on device.