Following a pair of posts (1, 2) where I mentioned the Zeiss 21mm king, I got in touch with Carl Zeiss to ask if they had plans to revive the Contax MM 2.8/21. You can see more of its handiwork here, here and here. The best 19-21mm lenses rated here.
A very nice, and nicely real person replied to my inquiry with the bad news:
Unfortunately it is not possible to release a new lens with the same lens design like the Distagon T* 2,8/21 MM (C/Y-mount) anymore. This lens contained glass types that are not compliant to the new RoHS guidelines anymore. [Carl Zeiss]
He did then go on to promise wide-angle goodies in the near future. Let's hope they put the same people on the task!