So, in returning to Ulduar with Khadgar and Magni, there are some old familiar faces that pop up - faceless ones, or, more to the point, faceless echoes.
Judging from that, it doesn't seem like Yogg is completely gone, though it could just be the faceless that are left over after his quote unquote, death.
Granted, he says the shadow of my CORPSE will taint the land, but I found it interesting there would be faceless echoes, and not just demons, as the scenario is based on that more than anything.
Thought it was worth mentioning. So. Is he really and truly dead? Is it just Y'shaarj like? Is it just faceless ones running amok with Yogg gone? Is it a hallucination entirely different?