According to ASRock support, support for bifurcation was added via BIOS update to other ASRock boards as well,
I asked about B550M Steel Legend and B550M Pro4 and was answered that:
"Both of B550M Steel Legend and B550M Pro4 board support "PCIe/GFX Lanes Configuration" feature."
I asked about B550M Steel Legend and B550M Pro4 and was answered that:
"Both of B550M Steel Legend and B550M Pro4 board support "PCIe/GFX Lanes Configuration" feature."