August 1st of 2007, Google dropped the supplemental tag from their search results. The main reason why they did this was because their supplemental index was fresher and deeper then it was when it first launched in 2003, plus, in my opinion, to reduce all the confusion is caused.
Google said back then:
The current system provides deeper and more continuous indexing. Additionally, we are indexing URLs with more parameters and are continuing to place fewer restrictions on the sites we crawl. As a result, Supplemental Results are fresher and more comprehensive than ever. We're also working towards showing more Supplemental Results by ensuring that every query is able to search the supplemental index, and expect to roll this out over the course of the summer.
Since then, there has not been much discussion around the supplemental results, due to it being out of sight, which makes it out of mind. But are SEOs and webmasters still confused and worried about this supplemental index?
A HighRankings Forum thread has discussion on that topic. It seems at least one person is still considered over this index. In fact, the SEO wants to know, which of her pages are in this index. Like I proposed when Google dropped the tag from the results, Google might want to consider adding a report to Google Webmaster Tools to aid the webmaster in knowing how many of their pages are in this index, this way they can work on getting them out.
Does it matter if those pages are out of the supplemental index? I think most SEOs would agree, yes, it matters. But now that we have our heads in the sand, we really don't know exactly how to improve those pages, outside of the "holistic" approach. :)
Here is a poll, are you still concerned about Google's supplemental index?
Forum discussion at HighRankings Forum.