Critical distinction, as merely changing OS notification settings will simply prevent notification content from being displayed on-screen.

Settings > Notifications > Messages > Show

      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.

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

Android > Settings > Notifications > Manage > Notification History

I've found other ways Signal can leak information, even with disappearing messages. It's not the total install-and-be-done privacy screen that some people think it is, and requires a little effort at the user end to fill in a few gaps.