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 [[ https://gerrit.wikimedia.org/g/mediawiki/extensions/ArticleCreationWorkflow/+/1aa0bc9c91ce17a35e19948a8adb9ef84e3743a4/extension.json | 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)
Proposed:
Create a new setting $wgNamespaceCreateProtection similar to $wgNamespaceProtection, which is user rights to create pages in a specific namespace.