Today I found a reproducible way to confirm ditherig 1.11 functioning correctly on Intel HD Graphics 6000 with an external monitor as opposed to a laptop panel.
I have various images for calibration, and some of them are gradients. I've been testing out a displayport to HDMI adapter to my old monitor that is 6-bit+FRC (temporal dithering) in the panel itself because HD streaming movies on Prime requires both Windows 10 and HDCP which means HDMI or DVI.
It appears that on the Windows drivers for Broadwell that I use, when the HDMI cable is set for YCbCr w/ IT Content off (I have not tested if the combo of both is required or just one or the other, will have to test this out later on) dithering is actually DISABLED by default in the Intel drivers and ditherig is not needed. I checked this in Windows 10, as well with the drivers it loads on a clean install, and it has the same behaviour so this is not an artifact of the older branch and build of the drivers I use on Windows 7.
Steps to reproduce: Load up a nice big greyscale gradient image in YCbCr w/ IT Content off. What you should see: evenly spaced banding in the gradient (this is correct for this mode). Load up ditherig 1.11. What you should see: no change (it starts with dithering off).. now, if you select spatial, the greyscale gradient will turn smooth, etc. If you select temporal, you will actually see moving ripples on the image because both the monitor and the card are creating an interference pattern by doing the same thing at the same time.
Unfortunately it is not possible to shut off temporal dithering in the hardware of the monitor itself through a method currently known to me. It is probably possible using a factory service mode of some kind but it's good to know that this method of connection + setting is confirmed to not have dithering on by default and if one gets a display that is a true 8 bit or 10 bit panel with a good backlight, then eyestrain past that point should be from other sources.