Your correct according to this tech site: AMD Ryzen Threadripper 3970X & 3960X CPU Specs, Prices Confirmed
It mentions that 1st and 2nd Generation Threadrippers will use TR4 socket while the new 3rd generation Threadrippers will use sTRX4 socket.
Yes, AMD's specs say they are Socket TRX4 and will use the TRX40 chipset. PCIe 4.0 required them to build a new chipset, and backwards compatibility would make little sense given the 2990WX already has 32 cores on the TR4 socket.
The way I see it it's just AMD not wanting to force manufacturers to figure out what ways to compromise their systems to allow PCIe 3.0 Threadrippers to work the way X570 boards are compromised when PCIe 3.0 Ryzen processors are used on them. These are ultra high end enthusiast platforms which will be packed to the gills with expansion cards and NVMe drives, you don't want to see what you see on the X570 boards.
As for where the 16 PCIe lanes went, there's 8 tied up to link the processor and chipset, and 8 reserved lanes reserved for chipset uplink, yielding 72 available PCIe 4.0 lanes.
black_zion, this is all very confusing. The AMD specifications say 12nm for the 3960X and 3970X but your graphics say 7nm. Specifications say Zen Core Architecture and the graphics say "Zen 2" Architecture. I could find no place in the specifications where it says TRX40 chip set. What happened to the 16 PCIe lanes - difference between first and second graphic? Weird. Enjoy, John.
Look at Black Zion image from AMD saying it is using a different Chipset where is says New TRX40 Platform with 88 PCIe 4.0 lanes. Doesn't mention being backwards compatible.
Besides the size of the manufacturing process has nothing to do with what motherboard socket it will fit.