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The Canon USA store has stock of the brand new Canon RF 400mm f/2.8L IS USM & Canon RF 600mm f/4L IS USM. Stock levels are very limited and these lenses will likely be difficult to purchase for the foreseeable future.
One of the cool features of the upcoming Canon EOS R3 is its ability to send more power to the autofocus motors to improve performance. The EF versions of these lenses are not capable of this feature.