BillionGraves.com Index is no longer on Ancestry.com. This is likely because of Ancestry.com's purchase of FindAGrave.com. It may also be the result of MyHeritage.com's involvement as a competitor, with BillionGraves.com. See this screen shot:
This is a somewhat unanticipated development of the continuing movement in the commercial online world of genealogical databases. You might recall that a website named Footnote.com was acquired by Ancestry.com some time ago. The original Footnote.com website purported to be in the process of digitizing a considerable number of documents from the U.S. National Archives. The number and variety of documents was impressive and was proving to be very useful. After the purchase, the name of the website was changed to Fold3.com and the focus of the website became military records only. Some of the more valuable records from the original site are much harder to use and find.
When a genealogy company is acquired or enters into a partnership with another company, the outcome is no different than any other such transaction in the commercial world. The acquired company can continue to exist either with or without it original management team, or the acquired company can cease to exist, either immediately or over time as it is absorbed into the acquiring company. There are several examples of each possible iteration.
It is almost inevitable that additional acquisitions and mergers will occur in the future. For the genealogists who are on the receiving end of these transactions, we will likely see similar movements of the collections from one company to another and as with the BillionGraves.com Index on Ancestry.com, some collections may change availability and location.