There's no slope restriction involved, and yes, I also tried the smoothing filter. No luck, strangely.
The dunes are just a separate PF (color) to displace, so I used the smoothed color (and also tried a separate line with 20m smallest scale and 0.5 color roughness) to get a smooth displacement (by normal), then a displacement to vector, multiplied the vector by 0.01/1/1 (and/or rotated 90ยบ by Z) and got my eastside alright.... but not smooth. I just don't understand what I'm doing wrong.
Even tried things like visualize normal, and adding compute terrain, or compute normal, but got nowhere. So I'm not using the eastfaces mask you're referring to, WAS, or any older 'get normal' setup. That wouldn't work here.
I will try what happens if I displace the dunes smoothly to begin with and add smaller ridges after greening the eastsides....
I would say this is an important feature to be had, to get whole mountain sides to be masked for vegetation/snow, etc, and not all smaller hills excluded.