Enable maintenance mode via code

maintenance-mode.php

<?php
/**
 * Maintenance Mode
 *
 * @package           MaintenanceMode
 * @author            Micah Wood
 * @copyright         Copyright 2022 by Micah Wood - All rights reserved.
 * @license           GPL2.0-or-later
 *
 * @wordpress-plugin
 * Plugin Name:       Maintenance Mode
 * Plugin URI:        https://gist.github.com/wpscholar/de814426102cb17b3cfc9328303f1575
 * Description:       Shows an "Under Maintenance" message to logged out users when plugin is active.
 * Version:           1.0
 * Requires PHP:      5.6
 * Requires at least: 5.0
 * Author:            Micah Wood
 * Author URI:        https://wpscholar.com
 * Text Domain:       maintenance-mode
 * Domain Path:       /languages
 * License:           GPL V2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 */
 
add_action(
	'get_header',
	function () {
		if ( ! current_user_can( 'manage_options' ) || ! is_user_logged_in() ) {
			wp_die( '<h1>Under Maintenance</h1><br />Website under planned maintenance. Please check back later.' );
		}
	}
);