I had a Z9 + lenses on loan from NPS Switzerland. I am a usual D6 sport photographer. Z9 is a NOGO for the time being. Main reason; low light performance.
D6 is a far superior camera compared to the D5, but felt out the general attention (Z-launch, COVID ?).
I agree generally with that. I too have the D6 and find its autofocus much nicer to use than the D5 and more reliable in sports situations than the Z8. The D6 has a lower pitched sound than the D5 and is less distracting. It probably fell out of potential customers' eyes because COVID prevented a lot of sports events from happening and some web sites that pushed mirrorless hard (such as dpreview) didn't review it, many people likely assumed it was not reviewed because it wasn't notable enough to review. Dpreview did complement the 1DX III because of its live view focusing abilities and seemed to be sorry that it was a DSLR. So it was pretty obvious why they didn't care to review the D6, because they wanted manufacturers and photographers to go mirrorless, start shooting videos (whether they wanted or not) and buy all new kit whether it is better for the job or not. I think the D6 thanks to its custom group area with human subject recognition is the easiest way to shoot sports for me, and the most reliable out of all the technologies I've used. The Z8 finds spectator eyes quite often, and can be stuck there especially at high frame rates rather than the athlete. This happens very easily when shooting at 15-20 fps while at slower rates it is less erratic in this area.
I also find the D6 more fun to shoot with, due to the optical viewfinder, and it's easier for me to do single frame at a time shooting with the intention of getting the highest rate of keepers, minimizing editing time and getting the most of my personality into the shots.
With the VR 200/2G II the Z8 (which is what I have instead of the Z9) does work very nicely though, with the f/2 aperture I can go in low light and the image quality at ISO 1250 is OK. But I still find the colours and tones more pleasing from the D6 at high ISO, so I use the D6 when using, e.g., the 300/2.8. I'm amazed how much negative response and discrediting comments I get when I say these things on some other forums.