Know what Supplemental Index actually mean.

What is Supplemental Indexing? 

Ok, let me explain in a very simple ways (less technical). Lets imagine Google has two databases where they keep the information of the websites. Lets name them as 1st database is called "MAIN INDEX"
2nd database is called "SUPPLEMENTAL INDEX"

Now when Google finds a website that is using Spam Search Engine Optimization Techniques, then that website will be kept in the SUPPLEMENTAL INDEX DATABASE.

Keeping the said website in the SUPPLEMENTAL INDEX DATABASE (because it was considered as spam), Google will not place much value to that particular web page. As a result, that supplemental webpage will have very fewer chances or very less chances to get returned search engine result unless that search is done only on the single specific domain that the page abides in.

Some helpful Links:

Supplemental index 

View your Google supplemental - SeoBook.com