What plane of motion is the sumo squat in?

If you look at what the body is doing during the motion you will see the posterior of the body traversing rearward and the knees moving in the anterior direction. This makes the movement a sagittal plane movement.

http://www.davedraper.com/blog/2008/05/21/sagittal-frontal-and-transverse-planes-planes-of-human-motion/