VTK wrapping of external code may have been removed from VTK but it is reimplemented for example in vtkDICOM by David Gobbi: https://github.com/dgobbi/vtk-dicom/blob/master/CMakeLists.txt. It belongs into VTK, since this will be needed by many other libraries, but for now we can copy-paste the solution of vtkDICOM. Probably @jcfr we’ll need your help with this.
vtkOpenVR is a very special case, because it is a remote module with its source code stored in the VTK main repository. This “neither here nor there” state does not make much sense for me, maybe it is like this for Ken’s convenience, and maybe it has been changed since VTK-8.2. I guess it will either become a proper remote module (like spline reslicer or dicom) or a regular built-in VTK module. If it becomes a built-in module we lose the ability to do updates for stable releases and we’ll be forced to ship third-party DLLs with Slicer core. These limitations are tolerable, but it would be better if vtkOpenVR would simply become a remote module.