The prototype of the Canon EF 600mm f/4L IS DO BR USM lens was shown at Canon EXPO in September 2015. Thanks to the diffractive optics inside, the new EF 600mm f/4L DO BR lens is substantially shorter than the current EF 600mm f/4L IS II USM lens (Amazon, B&H).
Besides this EF 600mm f/4L IS DO lens, Canon is also working on updates for several other super telephoto prime lenses. And all these new telephoto prime lenses are expected to be lighter, shorter and have updated coatings and IS systems.
Last year, Canon announced the EF 400mm f/4 DO IS II USM lens, it’s really amazing! Hope Canon will announce the new Canon EF 600mm f/4L IS DO lens in late 2017.
None is confirmed anyway so it’s best to take reports with a pinch of salt.