Each version of macOS adds new features and new incompatibilities with previous versions of macOS. Three years ago, we dropped support for 10.7 and earlier, as described here, so we could use features that were added starting with macOS 10.8.
The viewport drawing in Rhino V6 has been rewritten and upgraded to use modern OpenGL. Mac V6 will use the same drawing code, and this will require relatively modern GPUs and newer versions of OpenGL. If a Mac cannot run macOS 10.13, then it won’t be capable of running the new drawing code in Rhino V6.
Note: macOS 10.0 came out in 2001, 17 years ago, about the same time as Windows ME, and was at least as buggy. Also, macOS 10.0 was only for the PowerPC. Intel Macs were way in the future back then. There are no Macs these days that can even run 10.0.