Thanks a lot for all the answers. Makes perfect sense.
I must have gotten the RenderingContextOpenGL2 by “luck” in my earlier VTK 8 build due to some dependency.
Since RenderingContextOpenGL2, unlike e.g. RenderingVolumeOpenGL2, is not part of the Rendering group (should it be?), I guess I must enable that module explicitly to get it?
Edit: Running a VTK build now with:
-DVTK_GROUP_ENABLE_Rendering=YES \
-DVTK_GROUP_ENABLE_Imaging=YES \
-DVTK_GROUP_ENABLE_Views=YES \
-DVTK_GROUP_ENABLE_Qt=YES \
-DVTK_MODULE_ENABLE_VTK_RenderingContextOpenGL2=YES \
-DVTK_MODULE_ENABLE_VTK_RenderingFreeTypeFontConfig=YES \
to check. Then I think I should get it for sure.