It's not because that slot is gen 4, it's because the slot is run from the CPU instead of the chipset. Only the 11th gen CPUs have the PCI-e lanes to run the slot at all. It's the same on all of the z590 boards.Well, this sucks.
The Strix Z590-i has two m.2 slots, but since one of them is pcie4, its disabled if you use a 10th gen cpu.
Of course its the slot where I put my boot drive...