Using Noindex Header Tag Will Not Prevent Google From Viewing a Page
Including a ‘noindex X-Robots-Tag’ HTTP header directive on a sitemap file will not affect how Google is able to process the file. You can also include this directive on other documents such as CSS files, as it will not affect how Google views them, instead it will just prevent them from showing up in a web search.
Either Disallow Pages in Robots.txt or Noindex Not Both
Noindexing a page and blocking it in robots.txt will mean the noindex will not be seen, as Googlebot won’t be able to crawl it. Instead, John recommends using one or the other.
Noindex Thin Pages That Provide Value to Users on Site But Not in Search
Some pages on your site may have thin content so it won’t be as valuable to have them indexed and shown in search, but if they are useful to users navigating your website then you can noindex them rather than removing them.
Google Will Use Other Canonicalization Factors If the Canonical Is Noindex
Google would receive conflicting signals if a canonical points to a noindex page. John suggested that Google would rely on other canonicalization factors in this scenario to decide which page should be indexed, such as internal links.
Only Use Sitemap Files Temporarily for Serving Removed URLs to be Deindexed
Sitemap files are a good temporary solution for getting Google to crawl and deindex lists of removed URLs quickly. However, make sure these sitemaps aren’t being served to Google for too long.
Submit Subdirectories to URL Removal Tool to Get Around Individual URL Limits
The URL Removal Tool limits the number of individual URLs that can be submitted to be removed per day. To get around this, you can submit subdirectories to get entire sections of content removed from Google’s index.
Number of Noindexed Pages Has No Effect on Rankings or Site Quality
Having a lot of noindexed pages doesn’t affect rankings or how Google perceives a site’s quality. For example, many sites need to noindex private content that requires a user to log in to access.
Many-to-one Redirects & Noindexed Pages Are Sometimes Treated as Soft 404s
Noindexed pages and too many pages that redirect to one URL can both be treated as soft 404 errors by Google. Having soft 404s doesn’t impact the perceived quality of your website, but these pages won’t be crawled as frequently or indexed at all.
Avoid Using Google Tag Manager to Implement Critical Tags Like Noindex