The B550A has been out for months in OEM systems,since last year - they were being made side by side. They have partial PCI-E 4.0 support and work fine:
https://www.pcgamesn.com/amd/ryzen-b550a-motherboard-pcie-4-support
So do you think AMD is going to make a special B450 chipset chip for OEMs and one for DIY motherboards - this is highly unlikely. BTW,there were also NEW B450 motherboards launched after Zen2 was released - examples include the MSI Max range. So why don't those support it then?
The OEM systems since last year had B550A motherboards,instead of B450 motherboards. So are you now telling me AMD knowingly sold B450 motherboards for the last 6 months which were faulty,but OEMs got B450 motherboards(made by the same factories) which worked OK with PCI-E 4.0? Seems a stretch - its more likely many B450 motherboards will work OK,if an OEM rebadge works fine. They use the same ASMedia chipset. It's more like they couldn't fool OEMs that easily.
Also,the Soyo B550 motherboard uses a thin,reduced form factor mATX motherboard. If you have ever seen some of those types of motherboards,they are thin and tend to flex more. Probably less PCB layers than the better quality B450 and X470 motherboards. Do you actually think an Asus ROG Maximus X470 motherboard has less layers than a cheap OEM Soyo B550 motherboard with two SATA ports??
Remember,how last year the excuse for the X570 only having PCI-E 4.0 was due to signal integrity and retimers,etc. Funny how a few months later for OEMs,a rebadged B450 chipset could have certified partial PCI-E 4.0 support,and the B550 has support for both the main PCI-E 4.0 slot and M2 slot,with a basic chipset and a cheap PCB. Yet all the newer B450 motherboards released since last year can't do it. Sounds more like they spent a lot of money on the X570 chipset and realised PCI-E 4.0 could probably work on the ASMedia chipsets,but decided it would reduce X570 sales,so decided to make it entirely exclusive to the X570.
Plus nobody said that all B450/X470 motherboards needed compatible BIOSes,it only needed to be for the better ones. I still find it hilarious a £50 level quality Soyo OEM B550 can have PCI-E 4.0 support and the best X470 motherboards are not "good enough".
We will have to agree to disagree on this then!