Seriously, no, he didn’t die. That’s what the Chaos party line is (insofar as Chaos has a party line…). There is abundant evidence that he is alive, else the Astronomican would fail and the Soulbinding would not be possible, just to name a few examples.
On the other hand while he definitely isn’t “dead” he isn’t “alive” either, and it is somewhat unclear if he is conscious in his current state.
Well, those of us who have read the Horus Hersey novels for a start…
And yes, yes he does, that omission was my mistake.