It is on the edit form of each route under Delivery Settings > Delivery Routes & Methods (shown only with the Root Deliveries module; on by default). In the current version it decides which exported routes get a route status card on the Deliveries page: route status is looked up only for orders whose delivery method has the switch on. The order tables themselves list every home-delivery order for the date regardless of the switch, so turning it off does not hide orders from the page.