It was the TR Pro last year which finally started to support RDIMM. No one released an mATX version though, pity. The earlier platform did have an mATX version but only four DIMM slots, kinda pointless there.
(My laptop has 64GB RAM, so I do not think 256GB on the desktop is unreasonable.)
Since Threadrippers until very recently didn't support RDIMM, you could only put 256GB in them -- but even for that little you needed eight slots. Source: https://www.reddit.com/r/Amd/comments/6icdyo/amd_threadrippe...
It was the TR Pro last year which finally started to support RDIMM. No one released an mATX version though, pity. The earlier platform did have an mATX version but only four DIMM slots, kinda pointless there.
(My laptop has 64GB RAM, so I do not think 256GB on the desktop is unreasonable.)