Found this on reddit:
Not sure if legacy repro does anything for WMR, but ForceDisableDX11 uses the CPU to do reprojection, whereas if its set to false WMR uses the "new" GPU-based reprojection. If your game is GPU-bound, you might get smoother reprojection using the old CPU-based method, and vice versa if your game is CPU-bound.
I'm CPU bound, so i'll leave it as it is