Now we see through a glass, darkly...
- Apr 5, 2016
Canon has said that these lenses were not designed for RF. They have also said that they released the RF versions of these lenses by popular demand, but that this was not the original plan. Presumably that means that more telephotos that were planned are still being actively worked on.
There was a thread covering this on Canon Rumors, and here is the source:
As part of our ongoing series of executive interviews at major camera and lens companies, we talked recently with Go Tokura, Chief Executive of the Image Communication Business Operations at Canon.m.dpreview.com
Thanks, I hadn't seen that as I was busy with a large shooting assignment that week.
Based on the DP Review writer's response, I don't think it was unreasonable to ask if those lenses had been developed with the RF system in mind.
"It was surprising to learn from Mr. Tokura that the EF 400 F2.8 and 600mm F4 professional telephoto lenses were not developed with future native RF support in mind originally, although it does at least explain the absence of a control ring from the new RF-mount versions."