Multi monitor display worked perfectly for long time, long before "it works on intel, let's call it new standard" crew decided that flat shared framebuffer is the only way to go (apparently because of compositors and nothing else), something that also broke support for multi-gpu and especially heterogenous multi-gpu.
Zaphod mode is still available. nVidia pushed its TwinView solution while everybody else was using either the Zaphod mode or Xinerama. I don't think TwinView on Linux allowed any dynamic configuration. So, when the rest of the world switched to the RandR extension, every graphic card was able to dynamically add/remove monitors while nVidia users were stuck with a subpar solution.
