Direct yourself to Battle.net and you will notice that the World of Warcraft expansion Warlords of Draenor is no longer available for purchase. That's because, as noticed by Reddit and confirmed by Blizzard, it's now part of the base game.
It's an unannounced (as far as I know) development, but not entirely unexpected. Despite being the most recent World of Warcraft expansion, Warlords of Draenor is a year-and-a-half old, and Blizzard has already incorporated all the previous expansions—The Burning Crusade, Wrath of the Lich King, Cataclysm, and Mists of Pandaria—into the main game.
Unactivated Warlords of Draenor registration codes can no longer be redeemed through Battle.net, which wouldn't be a big deal (since all the content is in there now anyway), except that the level 90 character boost included with the stand-alone expansion is no longer available. A customer support rep recommended that players with unactivated codes contact Blizzard for assistance, and said separately that Blizzard should also be able to do something for people who purchased the expansion “a few days” ahead of the changeover.
The next World of Warcraft expansion, Legion, will be out in August.