Simple Multisite Maintenance Mode is a maintenance mode plugin for WordPress Multisite. It displays a maintenance message to anyone that isn’t a Super Admin (manage_network capability).
It supports drop-in theme templates for customizing the look of the maintenance message. Site administrators can customize their maintenance message by putting a custom
maintenance.php in their theme directory. Each site in the network can have its own custom maintenance page. If a custom template isn’t found in the theme directory, it displays a default message using the fallback template that comes with the plugin. The default template sends a proper 503 HTTP status code and Retry-After header to tell search engines the maintenance mode is temporary so they don’t index your maintenance mode message.
If you’re a developer and would like to contribute, the plugin is on Github.
Download Simple Multisite Maintenance Mode
If you need paid support for this plugin, get in touch.