Manual:$wgExemptFromUserRobotsControl

<translate> Robot policies</translate>: $wgExemptFromUserRobotsControl
An array of namespace keys in which the __INDEX__/__NOINDEX__ magic words will not function.
<translate> Introduced in version:</translate>	1.14.0
<translate> Removed in version:</translate>	<translate> still in use</translate>
<translate> Allowed values:</translate>	(array of integers)
<translate> Default value:</translate>	$wgContentNamespaces
<translate> Other settings:</translate> <translate> Alphabetical</translate> | <translate> By function</translate>

Details

An array of namespace keys in which the __INDEX__/__NOINDEX__ magic words will not function, so users can't decide whether pages in that namespace are indexed by search engines. If set to null, default to $wgContentNamespaces .

Example:

$wgExemptFromUserRobotsControl = [ NS_MAIN, NS_TALK, NS_PROJECT ];

Or, to switch it off entirely:

$wgExemptFromUserRobotsControl = [];