On android there are apps that let you see the history - i use NotiStar occasionally to see if i unwittingly dismissed important notifications. And i believe there are apps/settings that help you clear the history from the device.

But this is a reminder that these centralized notification infrastructure (FCM and APNs) store notification content (if the app is told to send content in it - signal with option enabled wouldn't send content) even if we clear local history these middleman still hold it

On Lineage Android, i see: Settings / Notifications / Notification History.

If you drop a settings widget on your home screen, it will let you choose a specific area, including notifications.

I don't know if the output is the complete database.