Send cancelled pending orders email notifications

pending_cancelled_send_an_email_notification.php

<?php

/* WooCommerce emails for cancelled pending orders */
add_action('woocommerce_order_status_pending_to_cancelled', 'pending_cancelled_send_an_email_notification', 10, 2 );
function pending_cancelled_send_an_email_notification( $order_id, $order ) {
	if ( version_compare( WC_VERSION, '3.0.8', '>=' ) ) {
		// Getting all WC_emails objects
		$email_notifications = WC()->mailer()->get_emails();
		// Sending the email
		$email_notifications['WC_Email_Cancelled_Order']->trigger( $order_id );
	}
}