TL;DR: It's available, feature-complete and not being improved as of end of 2016. Support consists of me answering forum posts if I'm not swamped and am in a reasonably good mood, which is not as frequent a combination as you (or I) might hope for.
It was a little more complicated than that; the EdX merger ended up not happening for a variety of reasons I'm going to be intentionally vague about. CB itself was supported and in development up through 2016Q4. At that point, while we knew we had some devoted users, the decision from the EngEdu department was that the staff for CB could
better serve Google's interests by working on different
internal projects. At that point, it went into non-maintenance mode. Which largely meant that we felt it was reasonably complete, didn't have an embarrassing number of bugs, and so while we weren't going to be working on it, there wasn't any harm in just leaving it as a free open-source project. AFAIK, that's the plan for the indefinite future. The cloud on the horizon is that Python2 support is going to get end-of-lifed on App Engine at some point, seeing as how Python2 is within weeks of being
officially at the end of its service life. Check for official Google comms to see if there's been a hard deadline announced w.r.t. App Engine and Python2; I am not an official source of truth on that. We did look briefly at the amount of effort to upgrade to Python3. While the code itself is probably in decent shape, it is _heavily_ dependent on a number of App Engine APIs that are also going away.