You can go to Google search and type in:

site:www.yoursite.com *** -sjpked

replacing ‘yoursite’ with your domain/site to see what pages are indexed in Google’s Supplemental Index.

If you see any important pages there you should check your whole linking structure of your site. Are these pages linked properly? Are they orphaned? Are they well positioned in your internal site architecture? If there are obvious interior linking problems with these pages, fix them.

It is also a good idea to see what percentage of your pages are in this index.

How To Calculate Your Supplemental Index Ratio

You can get your percentage of Supplemental results by dividing the number of pages in the supplemental index by the total number of pages in the main index.

site:www.yoursite.com *** -sjpked
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site:www.yoursite.com

full source: Google’s Supplemental  Index - What You Need To Know By Titus Hoskins