I thought about this a bit more and have changed my mind on how we should approach this issue. I would venture to guess that 90%+ of operating system specific bugs are caused by the underlying support libraries that Grasshopper depends on. These being System.Drawing, System.Windows.Forms, MonoMac, and Eto. McNeel is responsible for developing these libraries and we expect other 3rd party Mac plug-ins to also use the libraries.
Grasshopper is our opportunity to find and fix these bugs. If we don’t support all of the same OSX versions on Grasshopper that Mac Rhino runs on, then these bugs will eventually show up in 3rd party plug-ins and will be much more difficult for us to diagnose and fix.
In short, we need to have GH run on all the versions on OSX that Mac Rhino runs on.