> Strange, but it is missing support for 9005 Turin. How can this be (except of 400W+ models)?

I'm not a board design specialst, but it is not uncommon that boards get newer revisions to support next gen CPUs with compatible sockets. I suspect ASUS K14PA-U12 is Turin PCU eligible, but the vendor possibly chose not to support Turin CPUs on the board. If there was a new revision of the board, then it possibly could support new CPUs.

> What do you think, could there be some true hardware issue making it impossible to workaround it in BIOS and thus no chance for coreboot either?

May be hardware incompatibility, if there are some modifications required to the design to support both Genoa and Turin. On the BIOS/coreboot side, it is possible to support both Genoa and Turin, even with a single flash image (only if the flash size is at least 32MB). It is software that needs board porting and testing.

EDIT: ASUS K14PA-U12 has only 16MB flash, so it can run only one firmware type at a time. I.e. if you plug Genoa CPU, you have to flash Genoa compatible FW. If you plug Turin CPU, you would have to flash Turin compatible FW.

That's true. The PCIe lanes on EPYC server CPUs are divided into the main Gen5 links on the SERDES (like I explained in the post) and the other lower speed lanes (Gen3/Gen4) come from the Bonus links. It is shown on the figures in the post. These Bonus links are often used for devices with lower bandwidth, like BMC. This is also true for Gigabyte MZ33-AR1, so the NVMe M.2 disk is connected to the Bonus links working at Gen3 speeds, despite the board manual says: 1 x M.2 @Gen4 (which must be a vendor's mistake).