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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.
This is for the client display only, and not the iOS API interface as I'm discussing. It's not very plainly laid out in the docs, but one would assume any queuing of content into the notification system would be stored or cached if not cleared. There doesn't seem to be a way to have a client of that system to clear it's own data once it's in there, just cancel last notification.