ArticleCreationWorkflow helps users create new pages, intercepting article creation for new users and giving them wiki-configurable options such as using the Article wizard or creating a draft as a user subpage.
The extension adds a new user right called createpagemainns which permits users to create new pages in the main namespace.
This right is given by default to all users.
Users without this right are prohibited from created main namespace pages, and when they attempt to (by clicking on a red link, for example) they're redirected to the Article Creation Workflow landing page.
This extension is being used on one or more Wikimedia projects. This probably means that the extension is stable and works well enough to be used by such high-traffic websites. Look for this extension's name in Wikimedia's CommonSettings.php and InitialiseSettings.php configuration files to see where it's installed. A full list of the extensions installed on a particular wiki can be seen on the wiki's Special:Version page.