Crossroads I/O has been superseded by nanomsg: https://github.com/250bpm/nanomsg

It's still pre-alpha but has a ZeroMQ compat layer already.

I see that nanomsg is still C as opposed to C++.

https://news.ycombinator.com/item?id=3953434

Why dead, why superseded?

IIRC: Crossroads I/O was forked from ZeroMQ by its original author, who subsequently decided to start from a clean slate with nanomsg. His blog has some really good insights on various design decisions and "lessons learned" from ZeroMQ: http://250bpm.com/blog