The program to control Intel Graphics Dithering on Windows, called ditherig (note the mis-spelling) is now open source on GitHub under an MIT License.
The source code is available here: https://github.com/skawamoto0/ditherig. I want to thank the author for releasing the source code.
I came across the source code by finding this GitHub issue on Iris software, where the author points to that repo. Please take note the ditherig author on that issue mentions that another tool, igfxtweak, is no longer updated.
I'd be interested in doing two things to ditherig, now that source is available:
1) Making it run on a modern Linux distro like Ubuntu. The current source (although haven't look too closely) seems to depend on some Win32 APIs -- so cannot just compile it.
2) Last time I checked, ditherig only worked on a laptop screen, and not any external monitors plugged in. Not sure if anyone has seen it different.
Anyone wishing to share wisdom on either of those, I would appreciate it!