So that classes which extend WC_Abstract_Order can define custom statuses
specifically for their order type and have those used for validation in
WC_Abstract_Order::set_status() instead of only the order statuses defined
by wc_get_order_statuses().
For example, the subscription order type has a number of custom order statuses,
like 'wc-active' and 'wc-expired', which do not apply to orders but are valid
statuses for WC_Subscription objects, which extend WC_Abstract_Order.
7a7c0fbce6