Kappador was spot on there :)

During Mists of Pandaria, each item had multiple versions, one per difficulty (these are the ones showing as no longer available).
At the end of Mists, just before Warlords of Draenor was released that system changed, so there's only 1 item (item ID) and the difficulty setting is handled by bonus ids.

Xal'atoh, Desecrated Image of Gorehowl
So this is the current available version of the item.
On that page the Item Versions link is how you change from LFR, Normal, Heroic and Mythic versions.
ID stays the same, bonus changes for difficulty and item levels etc.

And it does still drop, it's just one of those items that seems to take time to get, especially when that's the one thing you're really looking for :)

Best of luck with it!