I guess the first thing is to explain what supplemental results were. In laymen terms supplemental results are pages that Google keeps in a separate index. They were identified in the SERPS as supplemental when those pages came up in a search. I’m sure Yahoo, MSN, and Ask have something similar. The difference is they are not as open with webmasters as Google is trying to be.
To me it was no big deal to have pages show as supplemental. In fact I have seen pages that were listed as supplemental out rank my pages which were were not supplemental. To those involved in the Search Engine Optimization Industry it was a big deal. Those people did not like having pages in the supplemental results. SEO’s saw the supplemental results as a road map telling them which pages on their site needed more work.
I’m not sure when Google started showing supplemental results. I think it was sometime in 2006. However, as Brett pointed out in his article 7 SEO Techniques That Google Smashed in 2007 Google has quit showing supplemental results in 2007. Man that caused an uproar.
At some point someone discovered a operator which they believed would show only supplemental pages. Brett mentions that in his article. The operator was “site: *** -sljktf” command (to show the supplemental index results).
Some people thought you could still discover supplemental pages with the operator, even after Google stopped showing supplemental results . Personally I don’t think the operator was broke. I don’t think it was ever accurate.