Allow a Single Variation of Category Pages to be Indexed
Google doesn’t currently have guidelines on indexing different versions of category pages, but is moving towards recommending allowing a single version to be indexed, such as a sort order, and the alternative variations with different filters and sort orders should be noindexed. If there are other specific versions of category pages which are important, you can allow the first page in the set to be indexed as well.
Google Will Ignore Links on Noindexed Pages Over Time
If pages are noindex, Google will ignore those links over time. If you have pages which are only linked from noindex pages then Google may not see the linked pages as important.
Google May Treat Noindex Pages as Soft 404
Google may treat a noindex page as a soft 404, which are equivalent in how they are treated in search results. If you want them to be re-indexed, you need to let Google know the pages have changed, such as submitting in a Sitemap with a last modified date.
Options For Out of Stock Items Include Noindexing, Returning a 404, Adding Schema or Redirecting to a Replacement
Out of stock items can be dealt with by specifying in HTML and schema to show it is not available. Alternatively, the page can be noindexed, return a 404 or be redirected to a replacement product.
Google Would View a Page Canonicalized to a Noindex URL as a Noindexed Page
If you have a canonical link pointing to a page that is noindexed, the page canonicalised to it would also be considered noindex. This is because Google would view it as a redirect to a noindex page and therefore drop it.
There is No Risk of a Noindex Signal Being Transferred to the Target Canonical Page
If a page is marked as noindex and also has a canonical link to an indexable page, there is no risk of the noindex signal being transferred to the target canonical page.
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.