I've been wondering about that as well. I do have the RF 16-28mm & 28-70mm f/2.8 that I use with the R8 in a small/light kit (I have an even smaller/lighter R50 based one). I've used the 70-200mm F4 with it and it is fine. Actually, the 70-200mm F/2.8 is compact enough that it can fit in my small kit bag. It is heavier, but compact enough to fit so I often use the 2.8 version.
I do agree with
@Exploreshootshare that it would have to be really compact and light, i.e., less than 500g like the 16-28mm and the 28-70mm. The 70-200mm F4 is only 695g, so even then the difference in weight (and probably size) wouldn't be that large, but for that focal length F2.8 vs F4 would make a difference. Also, the RF 70-200mm F2.8 is 1Kg, so the weight difference is considerable and I'd assume an RF 70-180mm F2.8 would be less than 50% the cost of the RF 70-200mm F2.8.
I did look into the RF 14-35mm F4, and perhaps that would have been the better option. The 14mm vs 16mm is a significant advantage and the 35mm vs 28mm is as well. The F2.8 vs F4 tends to be not that great of an advantage for me.