See also:

• https://www.mediawiki.org/wiki/Manual:$wgNamespaceProtection (configuration setting for user rights to edit pages in a specific namespace)
• createpagemainns - a right defined by ArticleCreationWorkflow to restrict users who can create pages in main namespace
  • Why this feature should exist in a MediaWiki extension instead of core?
  • By the way the whole ArticleCreationWorkflow should be replace with mechanism in core - see my concern in T204234 (this is out of scope of this task)
• property-create - user right in Wikibase to allow creation of property (there're no correnspoding item-create or lexeme-create user right)

Proposed:
Create a new setting $wgNamespaceCreateProtection similar to $wgNamespaceProtection, which is user rights to create pages in a specific namespace.