This feature was removed from MediaWiki core in version 1.39 (phab:rMWea1c106ea9a3) (after being deprecated in 1.34).
Please see NamespaceInfo.php for an alternative way to use this feature.
MWNamespace.php contains the MWNamespace class, a utility class with only static functions for dealing with namespaces that encodes all the "magic" behaviors of them based on index.
MWNamespace::getCanonicalNamespaces($rebuild) - Returns array of all defined namespaces with their canonical (English) names.
MWNamespace::getCanonicalName($index) - Returns the canonical (English) name for a given index
MWNamespace::getValidNamespaces() - Returns an array of the namespaces (by integer id) that exist on the wiki. Used primarily by the api in help documentation. This function calls getCanonicalNamespaces() without using the rebuild option and it then excludes the negative number namespaces, e.g. -1 (special page).
