Manual:$wgNamespaceProtection/en

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 and cannot be modified in $LocalSettings 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 the 'editinterface' right to those groups.

It's not possible to restrict read access to a certain namespace with $wgNamespaceProtection. (See )

Restricting editing of the main namespace
This restricts editing in the main namespace to people in a group that has the edit-main permission.

Disabling talk pages
Since "everyone" is not a core permission, literally everyone gets denied access (including sysops). Note that you'll still need to remove the tab through other methods (see: )