It's not dead yet. David Miller has been untiringly maintaing Clojure's CLR implementation for years now. Clojure CLR right now has feature parity with Clojure JVM. core.async is still alpha (it's also an external lib), I bet that once it reaches some stability David will add support for CLR in a weekend (as it has been usually).
Unfortunately that doesn't mean there is much community interest in the CLR port though, but David's relentless stewardship of the project is relentless.
I remember watching a Hickey talk where I think I recall him saying it was even before 2011, but the exact name escapes me at the moment. However, I was able to find this snippet from 2011:
Fogus: Clojure was once in parallel development on both the JVM and the CLR, why did you eventually decide to focus in on the former?
Hickey: I got tired of doing everything twice, and wanted instead to do twice as much.
Years before the first release of Clojure, Rich used to maintain a parallel build of Clojure for both platforms before concentrating on the JVM; but that is ancient history, and isn't relevant to ClojureCLR today.
ClojureCLR is a port maintained by David Miller, Rich doesn't work on it, so it isn't surprising that he hasn't made any comments about it.
Though it is true that the CLR implementation has much less adoption than the JVM or JS implementations.
Inference : ClojureCLR seems to be dead :(