We know Canon is working on the new DO lenses, but these lenses would be for the Canon RF mount, not the EF mount. The latest rumors suggest that the first Canon super telephoto lens for RF mount will be a diffractive optics (DO) lens.
Currently, we don’t the focal length for the first Canon RF mount DO lens, but a 500mm lens will a good start for Canon EOS R mirrorless system. In addition, a new Canon RF 300mm f/2.8L IS USM lens is in currently in development, but it will NOT be a consumer product anytime soon.
And for now, if you want to get the lightest super telephoto lens for your Canon EOS R mirrorless camera, the Canon EF 400mm f/4 DO IS II USM lens will be the right choice!
There are lots of RF mount lens “rumors” around the web, so just take it with a graon of salt.