I've just marked the current state of the master branch as version 2.34. The main changes are:
compareStates is no longer needed.
onHighlightComplete no longer works.
2.34 will be the last 'full' release on the 2.x branch. I will continue to bring out bugfix releases on that branch for at least two more months, but new work will, from now on, happen on version 3.
The first beta version of CodeMirror 3 also came out today. The jump to version 3 is mostly a result of some of the major work I did last month, that the community generously sponsored. Some of that work required incompatible API changes, and those changes landed in version 3 rather than the 2.x branch. The current beta has no known major problems (issue list for v3 milestone), but contains a lot of new code, and a serious overhaul of the old code, so I would not recommend using it in production yet.
I would be very thankful for any testing, of the editor in general and especially of the new features. I've written an upgrade guide that describes what changed, what's new, and how to adjust your code to it.