There are literally 3-4 motherboards that could be tested at 8000+, and probably only ASUS Apex right now is a guaranteed one ... so it's on the QVL. G.Skill typically picks ASUS Apex for their top RAM series. Sometimes Gene or Impact as 2-slot motherboards OC better. From MSI are usually 2-3 top models too, like Unify-X or Unify ITX. From Gigabyte is Tachyon, but it appears on the market with a huge delay, so it's barely ever on QVL. The same EVGA Dark. ASRock used to have OC Formula, later Aqua, but it wasn't released in Z790 (yet), and their other series are just average recently.
I'm quite surprised, as in the new series, most motherboard manufacturers declare a maximum RAM frequency that is close to the motherboard's maximum frequency. Like Gigabyte says 7600 is max, on QVL 7600 is max, and 7800 was a max boot in my case (but not fully stable). MSI says 8000, 8000 works, and 8200 can't boot (so far, maybe will figure out how to make it work). ASRock says 6800, 6800 is stable, and 6933 barely boots. At least this is what I see in my tests and around the web.
AMD mobos are a different story. 6400 is the max on most series. 6600-6666 is the max if you are really lucky with the CPU.