Max cable length is very short, if you insist on using cheap ribbon cables with the relatively ancient PCIe CEM slot as a connector. Or you can do PCIe 4.0 x8 over a 2M cable, if you use quality twinaxial cabling with more modern connectors, eg. https://www.serialcables.com/product-category/pcie4-oculink-...

I mean, there's the length covered by the standard, which is short, and then these longer cables. If they work, great, but I don't believe it's required by PCIe. 2m is also a fairly short cable, if not as short as a few inches.

I don't believe I've ever run across a section of the PCIe standard dictating cable or PCB trace length requirements. All I've ever seen are signal integrity requirements (ie. dB loss) and timing requirements. The timing requirements are not tight enough to preclude long cables, and while the signal loss requirements are chosen based on what's practical and affordable to achieve with a PCB, they don't directly concern themselves with setting minimum or maximum reach lengths.