The Google Supplemental Index is not going away, however, the tag that displays on supplemental results is.
Onl July 31st, Google announced that they will drop the Supplemental tag from showing in the SERPs.
…we’ve decided to stop labeling these URLs as “Supplemental Results.” Of course, you will continue to benefit from Google’s supplemental index being deeper and fresher.
Source : Supplemental goes mainstream
Since Google initiated the supplemental results in 2003 there has been a lot of confusion on their impact. Now there are a lot of complaints about this latest move to remove the label.
The best thing to do, forget about it. Either you are doing a good job and your pages are indexed, or, you need to work on your SEO. Getting hung up on something like the supplemental is a good way to lose your focus on what is important.
Focus on your content, and the technical aspects of good pages and the rest will take care of itself.