WooCommerce Custom Order Status Generator

<?php
/**
 * Register a custom WooCommerce order status: "Backorder" (wc-backorder).
 * HPOS-safe: register_post_status() and the wc_order_statuses filter work the same
 * whether High-Performance Order Storage is on or off.
 */

add_action( 'init', 'ea_register_order_status_backorder' );
function ea_register_order_status_backorder() {
	register_post_status( 'wc-backorder', array(
		'label'                     => _x( 'Backorder', 'Order status', 'woocommerce' ),
		'public'                    => false,
		'exclude_from_search'       => false,
		'show_in_admin_all_list'    => true,
		'show_in_admin_status_list' => true,
		'label_count'               => _n_noop( 'Backorder <span class="count">(%s)</span>', 'Backorder <span class="count">(%s)</span>', 'woocommerce' ),
	) );
}

add_filter( 'wc_order_statuses', 'ea_add_order_status_backorder' );
function ea_add_order_status_backorder( $order_statuses ) {
	$new_statuses = array();
	foreach ( $order_statuses as $status_key => $status_label ) {
		$new_statuses[ $status_key ] = $status_label;
		if ( 'wc-processing' === $status_key ) {
			$new_statuses['wc-backorder'] = _x( 'Backorder', 'Order status', 'woocommerce' );
		}
	}
	if ( ! isset( $new_statuses['wc-backorder'] ) ) {
		$new_statuses['wc-backorder'] = _x( 'Backorder', 'Order status', 'woocommerce' );
	}
	return $new_statuses;
}

add_filter( 'bulk_actions-edit-shop_order', 'ea_backorder_bulk_action' );
add_filter( 'bulk_actions-woocommerce_page_wc-orders', 'ea_backorder_bulk_action' );
function ea_backorder_bulk_action( $actions ) {
	$actions['mark_backorder'] = __( 'Change status to Backorder', 'woocommerce' );
	return $actions;
}

add_filter( 'handle_bulk_actions-edit-shop_order', 'ea_backorder_handle_bulk_action', 10, 3 );
add_filter( 'handle_bulk_actions-woocommerce_page_wc-orders', 'ea_backorder_handle_bulk_action', 10, 3 );
function ea_backorder_handle_bulk_action( $redirect_to, $action, $order_ids ) {
	if ( 'mark_backorder' !== $action ) {
		return $redirect_to;
	}
	foreach ( $order_ids as $order_id ) {
		$order = wc_get_order( $order_id );
		if ( $order ) {
			$order->update_status( 'backorder', __( 'Order status changed by bulk edit.', 'woocommerce' ), true );
		}
	}
	return add_query_arg( 'marked_backorder', count( $order_ids ), $redirect_to );
}

/**
 * Notify the customer when an order moves to "Backorder".
 */
add_action( 'woocommerce_order_status_backorder', 'ea_backorder_send_email_trigger', 10, 2 );
function ea_backorder_send_email_trigger( $order_id, $order ) {
	$email = WC()->mailer()->emails['WC_Email_Backorder'] ?? null;
	if ( $email ) {
		$email->trigger( $order_id, $order );
	}
}

add_filter( 'woocommerce_email_classes', 'ea_backorder_register_email_class' );
function ea_backorder_register_email_class( $email_classes ) {
	if ( ! class_exists( 'WC_Email_Backorder' ) ) {
		class WC_Email_Backorder extends WC_Email {
			public function __construct() {
				$this->id          = 'backorder_notification';
				$this->title       = __( 'Backorder', 'woocommerce' );
				$this->description = __( 'Sent to the customer when an order changes to Backorder.', 'woocommerce' );
				$this->customer_email = true;
				$this->template_html  = 'emails/customer-processing-order.php';
				$this->template_plain = 'emails/plain/customer-processing-order.php';
				$this->placeholders   = array( '{order_date}' => '', '{order_number}' => '' );
				parent::__construct();
				$this->recipient = '';
			}
			public function trigger( $order_id, $order = false ) {
				if ( $order_id && ! is_a( $order, 'WC_Order' ) ) {
					$order = wc_get_order( $order_id );
				}
				if ( ! $order ) {
					return;
				}
				$this->object                         = $order;
				$this->recipient                      = $order->get_billing_email();
				$this->placeholders['{order_date}']   = wc_format_datetime( $order->get_date_created() );
				$this->placeholders['{order_number}'] = $order->get_order_number();
				if ( ! $this->is_enabled() || ! $this->get_recipient() ) {
					return;
				}
				$this->send( $this->get_recipient(), $this->get_subject(), $this->get_content(), $this->get_headers(), $this->get_attachments() );
			}
			public function get_default_subject() {
				return 'Your {site_title} order is on backorder';
			}
			public function get_default_heading() {
				return 'Your order is on backorder';
			}
		}
	}
	$email_classes['WC_Email_Backorder'] = new WC_Email_Backorder();
	return $email_classes;
}