Feature summary (what you would like to be able to do and where):
This would enable us to restrict the namespaces in which pages could be created

Additional considerations

• The syntax to define multiple namespaces as comma-separated list could be improved to the array syntax used by preloadparams[]=
• Both local and English namespace names as well as numbers (T126882) could be supported
• We might even provide a dropdown to select the namespace from the list of allowed namespaces (if provided), similar to the search page

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

• Validation (T382682) when using a namespace that is not in the list
• Hard-code a single namespace= to be combined with prefix= if provided

• This could even trigger an error message that the user is not allowed to provide any namespace prefix in the page title since it will already be provides and cannot be changed

Benefits (why should this be implemented?):

• Some wikis don't want certain users to create pages in certain namespaces
• Usually, when providing an InputBox for page creation, the intention is to create it in a certain specific namespace