We were told that development for RF mount super telephoto lenses is underway and that internal documents show that an RF 500mm f/4L IS USM will be the first super telephoto lens for the RF mount.
In the past, we've also been told that an RF 300mm f/2.8L IS USM was also under development, though we have yet to see any RF related patents for super telephoto lens design.
Canon tends to announce super telephoto lenses in pairs, so an RF 300mm f/2.8L IS and RF 500mm f/4L IS being announced alongside each other wouldn't be a surprise.
No timeline was given for such lenses, and I wouldn't think RF mount super telephoto lenses would come until we get that EOS-1D level EOS R body, which is still a ways away.
Thanks Dr Grammar