As you know, 32GB UDIMMs have now come out, example:
But I cannot find any on pcpartpicker.
Both of these are already on the site.
I am really sorry about that! It turns out the compatibility filter was on, which didn't show the kit due to the ASRock x399 professional gaming motherboard stating that it only supports up to 128GB RAM. I contacted ASRock (listing the specific kit), and they replied that "MB does support 256GB, and confirm from our HQ."
Sorry for the inconvenience.