With containerisation , I think it is possible that all the bags are containerised and sent to the aircraft before the pax board so by the time boarding is complete the luggage container could be on board so if the pax fails to board the container has to be removed to get the bag.
DL’s app lets you set up bag tracking push notifications. At most airports and in most circumstances, the ‘Bag on Board’ message comes about 2/3 to 3/4 of the way through general boarding. Other airlines are typically going to be similar