We had around 100 pages indexed which were actually outdated, thin content and causing SEO keyword cannibalisation issues.
For administrative purposes we are unable to remove the pages (as need them for internal purposes) but we have marked them all with meta name="robots" content="noindex" tags in the header
Given they are no longer linked anywhere on our website, and not part of our Sitemap, how will Google discover them to read the NoIndex directive and remove them from its index?