Manual:$wgNamespaceProtection

Details
This setting allows a wiki to require special permissions to edit some namespaces. By default, the only restriction is that the MediaWiki namespace can only be edited by users with the  permission (by default: sysops).

The keys of the array are namespace numbers, and the values are simple arrays of permission names. If you list more than one permission for a given namespace, a user must have all of them to edit pages in that namespace.

Since 1.14, the MediaWiki: namespace is unconditionally protected to users with 'editinterface' right (same as in previous versions). This is set in Setup.php and cannot be modified in LocalSettings.php since otherwise it's too easy to set it incorrectly and leave the wiki insecure. If you want to allow other groups than sysops to edit the MediaWiki: namespace, then grant 'editinterface' right to those groups.