Canon News has uncovered a patent application for a quad pixel autofocus image sensor.
Canon News explains Japan Patent Application 2019041178:
This patent application from Canon deals with a quad pixel autofocus sensor. Right now Canon is using dual pixel autofocus sensors, but if you ever tried to use an EOS R or an EOS M in landscape orientation to focus on a horizontal line you'll quickly realize that the phase detect sensors just go in one direction, and have little sensitivity in the other 90 degrees offset direction.
This patent application indicates that Canon has split the pixel into 4 pieces, and also offsets the microlenses as you go further out from the center.
The sensor they are describing in this document seems to be a 20.7MP sensor, with 83 million focus detection points! The pixel size seems to be 4 micrometers, which would make that approximately 22mm on the width (5575×3725) or in other words an APS-C sized sensor.
It's possible that Canon is looking at decreasing pixel density and moving to quad pixel autofocus in the future. Let's call it QPAF.
Canon uses 180nm tech for its APS-C sensors that can incorporate copper wiring. This is probably fine for a 20mp image sensor. There would be a loss of efficiency splitting the pixels further, and may lead to Canon dropping the pixel count on APS-C sensors. This would only matter if we actually do see QPAF sensors in the future.