Yes, it is in use.
The mdl_notifications is where the "Moodle updates are available" messages from Moodle HQ are stored when updates are available to the newer versions. Those appear on the Site admin > Notifications page for admins.
If this table is empty, it means your Moodle is probably newly installed and has not yet checked to see if there are any updates available for its version or there are no updates yet. This is controlled by the Scheduled Task "Check for updates" \core\task\check_for_updates_task.