Originally Posted by KyadCK
Explain 3900X on B350 then. The board, and even the chipset, do not need to support PCI-e 4.0 for the CPU to work in the socket. It can run in 3.0, just like when you put Ryzen3 in a Pre-X570 AM4 board.
Power is not a valid argument either, as 7nm draws far less per-core of it than 14/12nm did, and nearly all of the X399 boards are severely overbuilt.
As a reminder, the rumor is that TX40 will be a 4-channel board that supports up to 32-cores. X399 is a 4-channel board that already supports 32 higher power consuming cores.
I was only guessing pcie4 might have been one reason for the decision to break support, and power was just an example of why other sockets had done so in the past. Now we can see some of the new socket's motherboard spec:
Looks like they've changed the rules for how pci-e lane pins are used, and routed them differently on the motherboard side after the socket.. I'd have to find the same slide for previous gen to compare better.