At the end of July Google announced that “Supplemental Results” labels will not longer appear. Basically, supplemental pages still exist as before. We just can't see the status of pages using command 'site:example.com'. Label “Supplemental Results” helped many webmasters and SEO specialists improve the quality of websites.
Fortunately, there is a way to determine if page is in supplemental results. There are a couple of useful commands below:
site:example.com/& – will show a list of pages in supplemental results
site:example.com/* – will show a list of non-supplemental pages
Hopefully, this information will help you find low quality web-pages that stay in supplemental results to improve them. Diagnosis is the first and most important step for improvement.