Yesterday, the Google Webmasters Twitter account mentioned that the rel=prev/next tag is no longer used by the Google index spider:
The rel=prev/next attributes were used to indicate paginated content to Google – for example, long forum threads broken up into multiple pages, slideshow-style articles, and so forth.
Search Engine Journal has a longer article about this here: https://www.searchenginejournal.com/google-stopped-supporting-relprev-next-in-search-indexing-years-ago/299689/
Google used to have a help page explaining the use of the attribute – it was located at https://support.google.com/webmasters/answer/1663744?hl=en , but is now deleted. You can see an older copy here:
This change doesn’t surprise me – it’s been clear for years now that Google is increasingly using ML/AI to “read” web pages and infer relationships between pages, instead of going by what the page says about itself. The information that it extracts can then be used to power knowledge panels on Google search pages.
Furthermore, I wouldn’t be surprised if Google finds pagination relationships to be less important than other signals. For example: if there’s a long forum thread discussing some controversial issue, every page of the forum thread is not equally important; perhaps Google wants to focus on only those pages with important information, or those that get linked to the most frequently.