Custom order status

add_action( 'init', function() {

	register_post_status( 'wc-backorder', [
		'label' => 'On backorder',
		'public' => true,
		'exclude_from_search' => false,
		'show_in_admin_all_list' => true,
		'show_in_admin_status_list' => true,
		'label_count' => _n_noop( 'On backorder (%s)', 'On backorder (%s)' )
	] );

} );

add_filter( 'wc_order_statuses', function( $order_statuses ) {

	$new_order_statuses = [];
	foreach( $order_statuses as $key => $status ) {
		$new_order_statuses[ $key ] = $status;
		if( 'wc-processing' === $key ) {
			$new_order_statuses['wc-backorder'] = 'On backorder';
		}
	}

	return $new_order_statuses;

} );

add_filter( 'woocommerce_reports_order_statuses', function( $statuses ) {

	$statuses[] = 'wc-backorder';

	return $statuses;

} );