A patent showcasing an RF 13-21mm f/2.8 optical formula has appeared in Japan.
Canon RF 13-21mm f/2.8 embodiment:
Zoom ratio: 1.58
Focal length: 13.40mm 17.30mm 21.20mm
F number: 2.88 2.88 2.88
Half angle of view: 58.23 51.35 45.58
Image height: 21.64mm 21.64mm 21.64mm
Lens length: 150.15mm 136.89mm 130.04mm
Back focus: 20.96mm 25.13mm 29.31mm
I doubt this specific optical formula will become a consumer product, but it is probably part of the development for an ultra-wide-angle RF mount lens that we expect to see sometime in 2020.