Oh wow, I saw a video of Krita using that, but didn’t realize it was removed. I wasn’t very excited since it seemed to be in its own little window (like a palette), and in the video I saw blue+yellow made a muted yellow instead of green! Which, come to think of it. . .makes perfect sense for CMYK. Did you see my links to the work by Scott Burns? He has a method to map virtually every single RGB color to a spectral “pigment”. It sounds perfectly possible if we had enough CPU 
The RYB patch is getting pretty interesting, I think. I’ve fixed a few more errors and the color blending seems to be very consistent.
@AnTi already mentioned a few issues with HCY that I’m not sure how to fix; there is very little desaturation of mixed pigments, and there are some odd blending intervals caused by the shifting of the hues- see image below.
That made me think that maybe the main issue was trying to contort the RGB color wheel into an RYB color wheel while still using RGB as the primary colors. So RYB solves that issue and it has desaturation, but it still retains all the other problems, which are pretty tremendous. Namely, everything else about MyPaint is RGB, particularly anything to do with opacity. So for the best RYB results you need to work on one layer, set brush opacity to 1, set hardness to 1, avoid use of Radius_by_random, and set pixel feathering to 0. All of those things introduce RGB blending and detracts from the RYB blending. It’s not terrible to leave them on but it can be noticeable.
I added a setting to control the way the desaturation works-- either accelerating additional desaturation or even reversing the process:
Anyway, I definitely feel RGB smudge is here to stay, but pigment blending might a place too for “painterly” effects. @achadwick had the good idea to make it a slider so you can switch from one way to the other so it’s not a major change to MyPaint.