The sad fact of the matter is that throughout their two year existencei, the X.IDIFF difficulty futures have not managed to attract significant trade volume.
For a long time I thought this is due to objective conditions in the market that make such derivative instruments unfeasible - even though some voices held the view that their structuring is simply not useful. Meanwhile wild swings in difficulty are a thing of the pastii, and while intrinsic difficulty in pricing such complex instruments remains, no observable improvement in X.IDIFF trade suggests to me that the continuation of this instrument is simply not useful.
Consequently, the contracts in question will cease to be traded on MPEx forthwith. They go to join the ranks of the other futures, where perhaps they'll be happier in repose than they were in the daily bustle of the marketplace. There are currently no open position - in fact no open position have existed in quite a while, so this measure doesn't significantly impact anyone.
I remain open to listing futures on MPEx, particularly for Bitcoin mining difficulty, should someone trustworthy desire to marketmake them. It is altogether unclear to me how such an instrument could be well structured, but I am open to proposals and suggestions. Perhaps a close approximation of an actually delivered future, where delivery takes the shape of newly minted coinbases - so one contract deliverable in say July 2015 results in a Bitcent paid to the holder's address out of a freshly minted block reward sometime during July the 15th, 2015. Perhaps something else - I am willing to work with the prospective MM in drafting something sensible. In no case however will MPEx list an undeliverable future.