The Intel CPU is more powerful.
Some ASRock X299 motherboards like "Fatal1ty X299 Professional Gaming i9" and "X299 Killer SLI/ac" claim to support Windows 8.x, but the provided drivers are actually not for Windows 8.x. The chipset driver is compatible with Windows Server 2012 R2 though, which is based on Windows 8.1; maybe the driver can be installed with some modifications.
Broadwell-E (best: i7-6950X) is the actual last architecture to support Windows 8.x. Its CPUs were released in May 2016, less than 4 years after the release of Windows 8 in August 2012, less than 3 years after the release of Windows 8.1 in August 2013, and over 3 years before the release of the last Windows 7-compatible CPU in October 2019.
Even the first generation of AM4 chipsets, A320/B350/X370, only supports Windows 7/10/11. The last high-end CPU before that generation happens to be the same as the last that supports XP, as AMD didn't make high-end desktop CPUs at all between 2013 and 2016.
The RTX 2080 Ti is between the R9 295 X2 and the RX 6950 XT in performance.
On the driver search page, the last generation that Nvidia advertised Windows 8.x compatiblity for is the GeForce 10 Series. However, the Windows 7 driver actually contains lines in the INF files for the 16 and 20 series of cards, so these cards should be supported without any driver modifications. The 30 series (and TITAN V, CMP 90HX, CMP 70HX) is only listed for Windows 7 and not 8.x though, and adding the lines to the INF files is still not enough to make it work, according to an MSFN thread.
A patched driver for GeForce RTX 30 series (best: 3090 Ti) is available.
According to the release notes, AMD Radeon™ R9 Fury, Radeon™ Pro Duo and Radeon™ RX 400 Series are only supported by Radeon Software Crimson ReLive Edition on Windows 7/8.1/10 64-bit. AMD Radeon™ RX 500 Series graphics are only supported by Radeon Software Crimson ReLive Edition on Windows 7/10 64-bit.
However, the last Windows 7 driver for RX 6950 XT has lines for Windows 8.1 in its inf files. A Reddit post suggests that it might not actually be compatible though.