While Signal does use Google’s notification system, my understanding is that they just use it as a way to wake up the app on the phone and have it check for new messages rather than sending any message content this way. The notification system that the operating system provides on the phone would still have access to the message data in the locally generated message notification with the actual content however, along with any apps that you give access to your notifications.
Notification management: Adds action buttons to notifications. For example, the action buttons could add directions to a place, help you track a package, or add a contact.
Supposedly Shreddit uses the archive of your data that you can request from Reddit to delete the stuff that isn’t currently viewable but I haven’t tried it first-hand (costs money). If you could feed the archive data into one of the other scripts mentioned in the comments here somehow, that would probably be the most thorough option, assuming you don’t hit an API rate limit.
While Signal does use Google’s notification system, my understanding is that they just use it as a way to wake up the app on the phone and have it check for new messages rather than sending any message content this way. The notification system that the operating system provides on the phone would still have access to the message data in the locally generated message notification with the actual content however, along with any apps that you give access to your notifications.
It may be something like Android System Intelligence or another similar service doing it: https://support.google.com/pixelphone/answer/12112173?hl=en