It would be great to be able to write something like 'latestFeatureCompatibilityVersion' and 'previousFeatureCompatibilityVersion', similar to 'latest' and 'last-stable' bin versions, instead of hard coding "3.2" and "3.4" everywhere in our JS code.

The code changes committed in SERVER-26828 should be considered in the solution. That code would benefit from a way to further determine whether the current release has a new feature compatibility version, because right now it's just a manual change between true and false.