AMD's new Threadripper processors leverage a new sTRX4 socket (and TRX40 motherboards), which is incompatible with the sTR4 socket used on first- and second-generation Threadripper processors. Despite the fact that sTRX4 has the same number of pins (4096) as TR4, they are electronically incompatible. AMD explains that the mapping of the pins to voltage/data is completely different, which means that you won't be plugging a Threadripper 3970X into an X399 motherboard or a Threadripper 2990WX into a TRX40 motherboard.
As AMD explains it (first spotted by WCCFTech):
We wanted to drive maximum performance for the 3rd Gen AMD Ryzen Threadripper processors and sTRX4 helps us do exactly that. The 3rd Gen Threadripper will have 88 total PCIe Gen 4 lanes with 72 usable (CPU+motherboard). The net of total versus usable is because we’re also increasing the CPU<->chipset link from 4x Gen4 to 8x Gen4—quadruple the bandwidth vs. 2nd Gen TR. Extra data pins between the chipset and CPU make this possible, so you’ll be able to hang more I/O off the motherboard at full performance.
AMD goes on to say that the switch to sTRX4 "sets us up nicely for future development and scalability of the Threadripper platform, both on a near- and long-term basis." Although it's good to hear that AMD is looking at keeping sTRX4 around for the long haul, we don't know exactly what the means in years. TR4 only lasted for two generations (Threadripper 1000, Threadripper 2000), so we'd hope that sTRX4 lasts as long, if not longer.
If it lasts for two generations, that should get us through Zen 4. It's possible that it could last through three generations (culminating with Zen 5), after which it might be beneficial from a performance standpoint to switch things up again to bulldoze any bottlenecks/roadblocks standing in the way.
While AMD's currently-announced third-generation sTRX4 Threadripper processors top out with 32 cores and 64 threads, it's rumored that a benchmark-crushing 64-core, 128-thread Threadripper 3990X is on the way to smack Intel Cascade Lake-X across the face.