Manual:$wgNamespacesWithSubpages

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 Help:Subpages

Enabling for a namespace

 * See also Manual:Namespace constants 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 :

Enabling for all namespaces
This assumes you have no more than 200 namespaces.

If you only wanted to enable subpages for all talk namespaces, you would use:

Again, this assumes you have no more than 200 namespaces.

Default value
The default of this setting has changed over the years. To be sure which one applies to your wiki check this setting in "DefaultSettings.php".