There are forks though. The only version i don't think that has a fork is GNOME 1 but... the code is out there (and there is an actively maintained GTK1-based toolkit that was posted here not too long ago, though you may need to make some modifications to the GNOME 1 code to work with it as IIRC it isn't backwards compatible).
People made CDE to work on modern systems and IIRC CDE wasn't even compatible with Linux when the code was first released.