That totally sucks I was so hoping it would be compatible and able to push up to 2.8:1 with the teleconverter / extender, as I love using my 2x teleconverter with the EF 100mm f/2.8L is usm macro lens I've got atm to push up to 2:199% sure it will not work with the current RF TC's.
2 Reasons why...
Canon's product page does not state compatibility (vs the other 2 big white specifically state they are).
On one of the review sites they had a picture showing a partial view of the mount side of the lens. Although it was hard to make out the distance of the 1st glass element clearly (in relation to the mount), it appeared that there wasn't near enough room to fit the TC (the new RF TC's go into the lens a significant amount).
I spent over an hour on the phone with one of Canons tech support people who was helping me with an issue with the new R6 I just got, and she tried to see what she could find for me looking through the information she had just received on the new RF version. But she couldn't find anything on if it would or would not be compatible