wp-maintenance-mode.php

<?php
/**
 * Plugin Name: wp-maintenance-mode.php
 * Description: Maintenance mode for WordPress - Originally from https://wordpress.stackexchange.com/questions/398037/maintenance-mode-excluding-site-administrators
 * Version: 1.0.0
 * Status: Complete
 * Type: mu-plugin
 */

function wp_maintenance_mode() {
   if (!current_user_can('administrator')) {
      $url = "your maintenance file url you want to display while is on maintenance";
      wp_redirect( $url );
      exit;
   }
}
add_action('get_header', 'wp_maintenance_mode');