It's used in many games :). Fixed Foveated Rendering, as without eye-tracking.
Do you think eye-tracked foveated rendering is on the horizon, and that it will be as good as it sounds? I remember "DeepRender" from Oculus Connect 5, which required rendering only 5% of pixels on each display, but that was years ago. I haven't heard about it since, as well as all the other fancy features they talked about that, for the most part, seem to have disappeared and weren't mentioned in the Cambria presentation. 😅
I have no idea. With GPU similar to what Quest has at the moment, with breaking rendering into such big tiles, it might be unusable but then - if the area is big enough, you may not be able to notice, as peripheral vision is quite blurry and wherever you look, you should get only the sharpest image.
There were some companies that did eye-tracking stuff for Quest. It should be fairly easy to test foveated rendering if one has access to this hardware.