Add WooCommerce order meta if there's a product on backorder

woocommerce_checkout_update_order_meta.php

<?php
add_action( 'woocommerce_checkout_update_order_meta', function( $order_id, $data ) {
	foreach ( WC()->cart->cart_contents as $item ) {
		if ( $item['data']->is_on_backorder() ) {
			$order = wc_get_order( $order_id );
			$order->update_meta_data( '_has_backorder_product', 1 );
			$order->save();
			break;
		}
	}
}, 10, 2 );