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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 don't use Play Services and still get push notifications from Signal, so they're clearly using an alternative implementation.
You might be getting pull notifications, that's generally the workaround for push notifications being disabled - it generally increases battery usage because it forces the app to stay open in the background.
Molly supports unified push
That would make sense.