The notification_log_items table has a JSON column called extra that allows you to store any value you want.
To fill up the extra column, add a function named logExtra to your notification and let it return an array that should be stored in the extra column when the notification gets logged.
useIlluminate\Notifications\Events\NotificationSending;
// in a notificationpublicfunctionlogExtra(NotificationSending $event): array
{
return ['extraKey' => 'extraValue'];
}