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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 think it depends on where they're turned off. In Signal itself you can change it to remove the message text from the notification, or a few other variations.
If you just have notifications off on your phone in general, the notification is still being generated by the app and could potentially be stored somewhere on a server before being filtered out by your phone.