multisite-mainsite-redirect.php

<?php
/*
 * Multisite Main-Site Redirect
 * @package     MultisiteMainSiteRedirect
 * @author      Bernhard Kau
 * @license     GPLv3
 *
 * @wordpress-plugin
 * Plugin Name: Multisite Main-Site Redirect
 * Plugin URI: https://kau-boys.de
 * Description: Redirect the main site of a multisite to another domain or sub-site
 * Version: 0.1
 * Author: Bernhard Kau
 * Author URI: https://kau-boys.de
 * License: GPLv3
 * License URI: http://www.gnu.org/licenses/gpl-3.0.txt
 */

function multisite_mainsite_redirect() {
	if ( is_main_site() && ! is_admin() && 'wp-login.php' != $GLOBALS['pagenow'] && ! defined( 'WP_CLI' ) ) {
		wp_redirect( 'http://example.com/' );
		exit();
	}
}

add_action( 'init', 'multisite_mainsite_redirect' );