First, a critical setting for Signal users:

"Signal’s settings include an option that prevents the actual message content from being previewed in notifications. However, it appears the defendant did not have that setting enabled, which, in turn, seemingly allowed the system to store the content in the database."

Second, how can I see this notification history?

Not sure if it's exactly the same, but I had to add a When notification arrives with <message>, do <action> event trigger in my Crank macOS app (https://lowtechguys.com/crank) so I can show you how to do it on macOS:

      HOURS=6
      EPOCH_DIFF=978307200
      SINCE=$(echo "$(date +%s) - $EPOCH_DIFF - $HOURS * 3600" | bc)

      sqlite3 ~/Library/Group\ Containers/group.com.apple.usernoted/db2/db \
        "SELECT r.delivered_date, COALESCE(a.identifier, 'unknown'), hex(r.data)
        FROM record r
        LEFT JOIN app a ON r.app_id = a.app_id
        WHERE r.delivered_date > $SINCE
        ORDER BY r.delivered_date ASC;" \
      | while IFS='|' read -r cfdate bundle hexdata; do
          date -r $(echo "$cfdate + $EPOCH_DIFF" | bc | cut -d. -f1) '+%Y-%m-%d %H:%M:%S'
          echo "  app: $bundle"
          echo "$hexdata" | xxd -r -p > /tmp/notif.plist
          plutil -p /tmp/notif.plist 2>/dev/null \
            | grep -E '"(titl|title|subt|subtitle|body|message)"' \
            | sed 's/^  */  /'
          echo "---"
      done

In recent years, filesystem paths for system services have started to converge for both macOS and iOS so I'm thinking with jailbreak you could get read access to that database and get the same data out of it.

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

So I wonder about this. The quote from the 404 media article [0] is:

“We learned that specifically on iPhones, if one’s settings in the Signal app allow for message notifications and previews to show up on the lock screen, [then] the iPhone will internally store those notifications/message previews in the internal memory of the device,” a supporter of the defendants who was taking notes during the trial told 404 Media

The default setting appears to be to only show notification preview when unlocked. Will that notification still be stored unencrypted in notification storage or is it in an encrypted store because it will preview after unlock?

It makes sense that any notification that previews on the lock screen would be unencrypted (including the case where it is encrypted but the encryption key is adjacently stored).

This all reads to me that this was a user induced OPSEC issue and Signal had the right defaults.

[0] https://archive.is/bSQhD#selection-619.0-622.0

On a Pixel, I can see some history by going to

Android > Settings > Notifications > Manage > Notification History

On android its quite easy. There is a page of a protocol address that has all notifications show. I used to have a shortcut to it. It has been years since I was on android.

But it was really useful each time I did not see a notification in time.

Edit: typo

You don't, at least not without forensics tools

I wonder how long does the system store those notifications