Protection is used to restrict the making of changes to a page (edit or move), the creation of a page, or the upload of a file.
MediaWiki handles existing and not-existing pages differently, because not-existing pages don't have a page ID. That's why protection information is stored in two different tables, and listing of such pages are separate.
A page can be protected with the protect API.
A list of protected pages can be retrieved with the allpages API, using the
A list of protected non-existing titles (pages which have been protected to prevent their creation) can be retrieved with the protectedtitles API.
Protection levels of a title can be retrieved with the info API, using