Thus the opencv-python package could maintain backwards compatibility by simply being a meta package that requires opencv-python-base and opencv-python-graphics. I'm not sure if this is easy / ...

opencv-python was recently bumped to 4.9.0.80 on pypi. The wheel for x64 macOS seems to indicate that it's built for macOS 10.16 or later and gets pulled in by pip as a compatible package.

OpenCV's real-time GUI interactions receive input from user through windows by creating trackbars, reading mouse movements and keyboard taps.