A patent application showing the optical formula for an EF 10mm f/2.8 prime lens has been filed in Japan.
Japan Patent Application 2018-233223:
Focal length: 10.30mm
F number: 2.88
The half angle of view: 64.54°
Image height: 21.64mm
Lens length: 124.58mm
Back focus: 38.00mm
It looks like this patent may have been a part of the EF 11-24mm f/4L USM development.