The correct setting here is $wgExemptFromUserRobotsControl.

Notice the "If set to null, default to $wgContentNamespaces." part. It describes the MediaWiki behavior, and not our configuration, as we do this:

$wgExemptFromUserRobotsControl = array_merge( $wgContentNamespaces, $wmgExemptFromUserRobotsControlExtra );

Change 321713 had a related patch set uploaded (by Dereckson):
Allow NOINDEX on all namespaces on meta.

https://gerrit.wikimedia.org/r/321713

@MarcoAurelio Could you discuss the issue on https://meta.wikimedia.org/wiki/Meta:Babel?

Alternatively we could just declare NS0 to not be content for meta, which would be more accurate perhaps?

In T150245#2796907, @Jdforrester-WMF wrote:

Alternatively we could just declare NS0 to not be content for meta, which would be more accurate perhaps?

Content namespaces have a very large scope: Special:Random, visual editor

To modify such a used setting for one identified need doesn't seem valuable. Yes, we can add the NS0 back in each configuration parameter, but that's more efficient to fix the namespaces.

See https://gerrit.wikimedia.org/r/321712 for the offered solution.

In T150245#2796846, @Dereckson wrote:

@MarcoAurelio Could you discuss the issue on https://meta.wikimedia.org/wiki/Meta:Babel?

Sure, done: https://meta.wikimedia.org/?diff=16067526

The reason __NOINDEX__ doesn't work on content pages by default is that we were concerned that this magic word could be misused or abused on certain wikis. For example, imagine a user adding a __NOINDEX__ tag to a prominent article such as https://en.wikipedia.org/wiki/Barack_Obama, intentionally via vandalism or unintentionally via an accidental template transclusion. We didn't and don't want these pages to be go missing from search engine indices.

Meta-Wiki is obviously different than a Wikipedia. Is anyone concerned about vandalism or misuse of the magic word on Meta-Wiki? If not, this sounds fine to me.

@Dereckson It looks like there are no concerns for this. Can you arrange to have this deployed? Thank you.

No opposition.

Change 321713 merged by jenkins-bot:
Allow NOINDEX on all namespaces on meta.

https://gerrit.wikimedia.org/r/321713

Mentioned in SAL (#wikimedia-operations) [2016-11-29T20:05:04Z] <thcipriani@tin> Synchronized wmf-config/InitialiseSettings.php: SWAT: [[gerrit:321713|Allow NOINDEX on all namespaces on meta]] (T150245) (duration: 00m 44s)