Google May Take Longer to Process Redirects For Noindexed Pages
Google crawls noindexed pages less frequently. If a redirect is set up for a noindexed page, Google may take longer to process this because it is being crawled less frequently.
Noindexing Some Pages with Hreflang Tags Won’t Affect Whole Configuration
It’s fine to noindex some pages within a set which have hreflang tags implemented. The noindexed URL will simply be removed from the set that Google uses, without affecting the other pages.
AMPs Which Canonicalise to Noindexed Pages Won’t be Valid
If a page is noindexed which is connected to an AMP equivalent, then the rel=amphtml link will be dropped too and the AMP won’t be valid. If the AMP is noindexed then it will be removed from the index and the traditional page will be kept.
Don’t Rely on Unsupported Robots Directives in Robots.txt Being Respected By Google
Don’t rely on noindex directives in robots.txt as they are aren’t officially supported by Google. John says it’s fine to use robots directives in robots.txt, but make sure you have a backup in case they don’t work.
A Sitemap is the Best Way for Google to Quickly Process Noindex at Scale
Make sure the pages you’ve added a noindex tag to are included in a sitemap file with the last modified date to ensure Google picks these up as quickly as possible. Make sure last modified dates are realistic and aren’t the same for every page as this looks artificial to Google.
Google Eventually Treats Noindexed Pages as Soft 404s
Google treats noindexed pages as soft 404s after a period of time, as the page is ignored and essentially doesn’t exist in their eyes.
Videos Blocking Googlebot May Still be Crawled and Indexed
Blocking Googlebot from crawling a video may still result in a video snippet appearing in search if the video file is embedded from a different location, if some Google datacentres haven’t yet seen the updated version or if the video URL has parameters attached.
Noindex & 410 Pages Are Removed Faster Than 404
Noindex and 410 remove pages from Google’s index at about the same speed, and both are slightly quicker than using a 404.