Hi I did try to install LTSB on system with 1950x and it hang up on start (showing windows logo, never get to a points animation). I did try to slipstream KB 4035631 update (tested on older system OK), but it no helped at all. Only way to get it working is installing it on different system, make all updates, and transfer it to Threadripper.
Found unexpected solution: I use Gigabyte X399 and they released new BIOS F3d with description: -Fix unable to install Win10-64bit RS1 OS issue I'll test later, if bare ISO will work (tested with KB4035631 and 4034658 slipstreamed)