The chest piece was the culmination of a series of quests specific to Warlocks. The quests, like all other vanilla class quests, was removed in 4.0.1. (Despite what the airhead editors at Wowpedia think, 4.0.1 was indeed the patch that implemented changes to classic zones. -_- )
See obsolete quest http://www.wowdb.com/quests/4786-the-completed-robe.