Manual:$wgNamespacesWithSubpages/en

Details
The variable holds an array indicating which namespaces allow sub-pages. The array is indexed by the numeric namespace ID, and each array value is either 0 (no sub-pages) or 1 (sub-pages allowed). A categoryspace (namespace 14) subpage will work like any other category.

See.

Enabling for a namespace

 * See also for a list of the   codes.

The normal way to enable subpages for a given namespace is to edit the  and insert the following:

Disabling for a namespace
Disable subpages in a namespace (where it is enabled by default). For example  has subpages enabled by default in MediaWiki. To disable them, set :