WooCommerce - Redirect external products offsite (disable single listings)

functions.php

<?php // Do not include this if already open!

/**
 * Code goes in theme functions.php.
 */
add_action( 'template_redirect', 'redirect_external_products' );

function redirect_external_products() {
	global $post;

	if ( is_singular( 'product' ) && ! empty( $post ) && ( $product = wc_get_product( $post ) ) && $product->is_type( 'external' ) ) {
		wp_redirect( $product->get_product_url() );
		exit;
	}
}