It is still used by many things but in ways you might not expect. For example:
Code:
Xvnc -inetd -query localhost -once
Code:
Xrdp -inetd -query localhost -once
I believe even NoMachine uses it. XDMCP is one of the only ways to handle multiple remote sessions. Yes, X11 forwarding is less used but XDMCP is still quite common.
Don't get me wrong. I am not actually that religious about X11. In fact, since the dawn of time I have had terrible experiences getting it working with the correct resolution (especially when widescreen laptops were becoming the norm; 915resolution was dark times indeed). However I will be a little bit baffled if the Linux kids adopt a system that works terribly over the network and enterprise environment in this day and age just so they can have their fancy themes and their games.
Not a fantastic argument either but for digital preservation; the RDP server in Hydra (Windows NT 4.0 Terminal Server) has allowed this operating system to still be very usable even fully emulated in Qemu because the graphical calls can be passed out to be rendered on the native host. Separating the graphics system across a network brings fantastic benefits for emulation and keeping software alive. "Bash on Windows" would have also been more effective had the packages supported an X11 server like Xming rather than that Mir crap.