On WMF wikis, $wgExemptFromUserRobotsControl is set to include all content namespaces (e.g. main/article namespace). This means that the __INDEX__ and __NOINDEX__ magic words do not work on pages in those namespaces, they have no effect on search engine indexing.

However, the "Let this page be indexed by search engines" in Advanced settings in the page editor is still shown. This is confusing because editors who use this think they are changing how search engines index the page, but the magic word that gets saved to the page does nothing.