Manual:$wgArticleRobotPolicies

Robot policies: $wgArticleRobotPolicies
Allow customisation of robot policies on a per-page basis.
Introduced in version:	1.11.0 (r22615)
Removed in version:	Still in use
Allowed values:	(each entry indicates a page name and a text string)
Default value:	`[]`
Other settings: Alphabetical \| By function

Details

Allows you to set the robot policies for specific pages.

To get the meta element <meta name="robots" content="noindex,follow" /> for "Main Page" you will need to set:

$wgArticleRobotPolicies = [
    'Main Page' => 'noindex'
];