Is RhinoCommon still Open Source?
It was for Rhino 5: https://github.com/mcneel/rhinocommon
(I know, most of it is a wrapper around the native dlls.
But it would still be helpful to be able to read the source.)
The source is moving to the rhino3dm repository. We will most likely update the RhinoCommon repository to just contain a link to this new location.