It turns out xdmx currently crashes when any client attempts to use GL and it has been in such state for about 14 years. There was a patch by Adam to fix the problem 4 years ago, but it never got merged. The last activity on any bugs referring to xdmx has been more than 4 years ago.
Given such situation, I find it unlikely that anyone is still using xdmx and just having the code is a drain of resources.
Therefore I propose to drop xdmx not only from master branch, but also from X server 21.1. I think this is good enough reason to delay the RC1 of X server 21.1 by some more time.