CMIS came along aiming to standardize interoperability around content. It came to supercede JCR (which is pretty much dead for a ages now). I have never been a fan and always found CMIS cumbersome to work with – avoided it when I was given the choice. JavaScript folks did not care about it, and the JSON bindings introduced at v1.1 didn’t change that.
ECM vendors are pitching pure JavaScript libraries these days, and it seems they no longer care about CMIS either. I have not seen one of them pitching or recommending it for a while. However, would be fair to drop a few lines and guide the people who jumped on the bandwagon. I think it may be time to discourage CMIS usage.
Personally, I’d like to know what actually killed CMIS it. Maybe it was just what I found as a developer – the cumbersome usage and the fact that implementations where not as interchangeable as one might have expected.