About PCIe Bifurcation
PCIe bifurcation refers to splitting 1 PCIe Port into 2 or more with smaller lane width. This is a function of the CPU in most cases. Most recent CPUs support this. Single most important factor is BIOS/UEFI support.
By default lots of mainstream ATX Mainboards actually use PCIe bifurcation to either provide one x16 Slot or two x8 Slots for SLI/Crossfire via simple signal switches. Downside of this setup is that even if one device is idle, the other one is limited to x8 speed. The more expensive Mainboards may use often so called "PEX" or "PLX" Chips, which are actually PCIe packet switches that can distribute the upstream x16 Port into two or more downstream Ports, thus enabling a single card to get all x16 bandwidth, if the other ones are idle. These chips are also commonly found on dual GPU Cards back when those were actually a thing, however the reason for their use would actually be compatibility, not bandwidth. Even high end GPU performance doesn't suffer from reducing bandwidth from x16 to x8 in most cases.
The most common use for bifurcation risers would be mini-ITX mainboards with 2 or more Add-In Cards, or ATX boards with M.2 NVME Risers to support multiple SSD devices in 1 Slot.
|Asrock x99e-itx/ac||x8x8, x8x4x4, x4x4x8, x4x4x4x4||tested working well with linked official Asrock beta BIOS|
x8x8, x8x4x4, x4x4x8, x4x4x4x4
|not tested, modes as per BIOS dump||???|
For several generations Intel Mainstream CPUs have been supporting x16, x8x8 and x8x4x4 configurations.
For several generations Intel (on its 28 and 40 Lane CPUs) has supported x16, x8x8, x8x4x4, x4x4x8 and x4x4x4x4 as well as x8 and x4x4 configurations.
Ryzen CPUs have 20 CPU Lanes. Aparently Bifurcation to x8x8 for the x16 Port is supported.