Besides VMS, certainly those using HP-UX, IBM AIX, SCO UnixWare (or whatever it’s called nowadays), HP/Compaq Tru64 UNIX (formerly known as Digital UNIX, originally and internally still also known as “OSF/1”), Solaris <=10 and so forth, via either remote X or direct graphics output.
That reminds me, I almost forgot about it, there’s also the OpenCDE initiative nowadays! It’s not based on CDE, code-wise, but it’s rather a recreation of it. I do think it does rely on Motif, or LessTif rather. I successfully built it once, it seems to work okay! Below are some screenshots (from the site).
(As you can probably tell, they’re going after the Solaris look and feel mostly. You can, of course, customize that to be more DECwindows, HP-UX or AIX-like! )
What about XFCE, Blackbox, and Fluxbox? People still use those right?
I don’t know, I myself haven’t used Linux on the desktop much. The last time I seriously did, well over ten years ago (with proper DRI/DRM enablement), I used mostly AfterStep, Window Maker, MWM and some others.