AFAIK, the SoC root PCIe complex since Zen 1 supported both 8x/8x and 4x/4x/4x/4x bifurcation, just like the IMC supported ECC. This does not mean that the feature will be implemented though.
While it would be lovely for AMD to support a proper Xeon E competitor, as mentioned by many voices here before it’s a relatively small slice of the market. AMD probably doesn’t have enough resources to attack every segment comprehensively like Intel as much as we, and I would love to have an official AM4 based Xeon E competitor.
AMD Chipset slides pointed out that only X series Chipsets allowed for bifurcation to 8x/8x, while the B and A series are 16x only. I don't recall a single time that AMD said that consumer Ryzen can do 4x/4x/4x/4x, albeit I know that the silicon can do it. Getting the Chipset involved is like what Intel does, which only allows bifurcation to 8x/8x or 8x/4x/4x on the Z series Chipsets and some of the Q and C high end models. It is considered a "Chipset controlled Processor feature" by Intel. On AMD case... we don't really know because it seems that the marketing slides aren't enforced at the Hardware level, they do it as a "me too". Same with ECC, it is on the die, it isn't disabled, but it isn't oficially validated, either. So you don't even know what the Processor can do, or not, unless a Motherboard manufacturer decides to support it on its own so you at least have proof that it does work.
I think that to tackle the Xeon E line you just need more availability of Ryzen Pro and more Motherboards with BMC/IPMI. But as a DIY, the blame is on Motherboard manufacturers for not trying to find newer niches. AsRock is pretty much the only one that gets out unique designs.
