This index covers any images that had been previously covered by the original pixiv index, but were deleted between 2008 and 2011. At the moment it is searched at the same time as the 5_pixiv index, and is included with the 999 search.
Its worth noting that this index is really just for basic coverage... There are certain cases where it could come in handy to know which member, the title, or what ID a deleted image was assigned, but beyond that it wont be much help. The links to where the images would have been on pixiv will generally be dead, and in many cases the members will be gone as well.