Setting ‘noindex’ for a URL means that Google no longer evaluates the links pointing to this page.
Before you set a page to ‘noindex’, you should check whether there are backlinks pointing to the page, because these links are no longer evaluated by Google’s ‘noindex’.
In the May 14 Google Search Central SEO Offices Hours, John Mueller explained that Google always considers links between canonical URLs. If one of the URLs is no longer canonical, for example because it was set to ‘noindex’, then this connection no longer exists. This can also happen if a page suddenly returns a 404 error. These are all reasons for Google to assume that the page in question no longer exists and links to it are no longer relevant.
This also makes sense, Müller continues, because if someone recommends something specific on a website and this information is suddenly no longer available, then this recommendation loses its meaning.
It is different if someone recommends a website as a whole by linking to the homepage. This recommendation will remain as long as the homepage exists.