Magento 2 - Show custom maintenance page instead of "Unable to proceed: the maintenance mode is enabled"

Question

If I switch my store to maintenance mode, then I get so see this page:

1 exception(s):
Exception #0 (Exception): Unable to proceed: the maintenance mode is enabled. 

Exception #0 (Exception): Unable to proceed: the maintenance mode is enabled. 
<pre>#1 Magento\Framework\App\Bootstrap->run(&Magento\Framework\App\Http\Interceptor#000000001f0deaee00000000184ccfc2#) called at [pub/index.php:45]
</pre>

How can I show a custom CMS Site or HTML Site instead?

Answer 1

When switching to the maintenance mode and developer mode is enabled, it will show the below message instead of 503 - Service Temporarily Unavailable page. The error message returns from the function assertMaintenance() from the file /var/www/html/magento/vendor/magento/framework/App/Bootstrap.php

1 exception(s):
Exception #0 (Exception): Unable to proceed: the maintenance mode is enabled. 

Exception #0 (Exception): Unable to proceed: the maintenance mode is enabled. 
<pre>#1 Magento\Framework\App\Bootstrap->run(&Magento\Framework\App\Http\Interceptor#000000001f0deaee00000000184ccfc2#) called at [pub/index.php:45]
</pre>

If you switched to the default or production mode from developer mode then it will show the 503 error message page (/var/www/html/magento/pub/errors/default/503.phtml)

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
?>

<h1>Service Temporarily Unavailable</h1>
<p>
    The server is temporarily unable to service your request due to maintenance downtime or capacity problems.
    Please try again later.
</p>

You can customize HTML of the 503.phtml page as per your requirement.

Hope this helps!