NUnit as a project has a tradition of keeping backwards compatibility. It ensures that even if you stay on a earlier version of Visual Studio, .net FrameWork, or whatever it is, NUnit should continue to work even with upgrades of NUnit.
So also for the NUnitAdapter. It should work on any version of Visual Studio after the 2012 version. We have managed to maintain that. However, over time it has become harder and harder to verify the compatibility. Microsoft has increased their release pace, and the number of versions and variations of Visual Studio has subsequently increased tremendously over the last years. We have not found an easy way to test all the possible variations, so have had to do a set of minimum tests, then respond quickly when someone raises an issue. Fortunately, there have not been many of those. We rely to a large degree on peer reviews using pull requests, which have weeded out most of these issues.