Astra Sites `wp-url` script dependency fix for old WordPress versions.

backward-compatibility.php

<?php
/**
 * Astra Sites `wp-url` script dependency fix for old WordPress versions.
 * 
 * @todo Change the `prefix_` and with your own unique prefix.
 * 
 * @since 1.0.0
 */
if( ! function_exists( 'prefix_backward_compatibility' ) ) :
	function prefix_backward_compatibility() {

		global $wp_version;

		// If version is above 4.9 then don't execute the code and return.
		if( version_compare($wp_version, '4.9', '>') ) {
			return;
		}

		// De-register.
		wp_deregister_script( 'astra-sites-admin-page' );

		// Enqueue.
		wp_enqueue_script( 'astra-sites-admin-page', ASTRA_SITES_URI . 'inc/assets/js/admin-page.js', array( 'jquery', 'wp-util', 'updates' ), ASTRA_SITES_VER, true );
	}
	add_action( 'admin_enqueue_scripts', 'prefix_backward_compatibility', 11 );
endif;