With today’s PTR build, Blizzard seems to be changing some data entries for Siege of Orgrimmar items, including Heirloom items from Garrosh Hellscream.
While these changes don’t confirm anything, it is possible that it is a change to prepare the Mists of Pandaria raid to be the new Timewalking raid. Making Mists of Pandaria the next Timewalking raid would be the perfect opportunity for a new source of the now unobtainable Garrosh Hellscream Heirloom weapons.