What is the Google Supplemental Index?
The Google Supplemental Index was a system used by Google to catalogue lower-quality or less significant web pages.
Introduced around 2003, it was a separate index from the main one, typically containing pages that had fewer inbound links, were infrequently updated, had duplicate content, or were considered less relevant or less important by Google's ranking algorithms.
Web pages that were part of the Supplemental Index would usually appear less frequently in search results, hence why website owners and SEO specialists aimed to avoid their pages being included in this index.
However, it is important to note that Google discontinued the public labelling of the Supplemental Index in 2007.
Today, Google's indexing process has evolved significantly, so the term "Supplemental Index" is largely outdated and not in current usage.