This feature does not exist, but you can easily implement it yourself, e.g.
in your site.php blueprint add a field maintenance:
maintenance:
label: Maintenance
type: checkbox
text: Enable Maintenance
Then add a new invisible page called maintenance with a text file called maintenance.txt and a maintenance.php template. Add the text you want to display when in maintenance mode. Alternatively, you can use a router to achieve the same.
Then in your header snippet add this:
<?php if($site->maintenance()->isTrue() and $page->uid() != 'maintenance') { go('maintenance');} ?>