The official stance is that he is dead for now. As the mythic ending (i.e. Archimonde dying in the nether) was intended to be the actual ending. However due to the miscommunication between the encounter & story devs and the cinematic devs, Archimonde ended up dying on AU Draenor. Therefore Blizzard has stated that they can simply de-canonize the mythic ending if they want to bring him back.