I believe that statement (the first half, anyway) is simply not true. X11 is capable of displaying different windows at different color depths. Look at the "int depth" parameter to XCreateWindow, or run xwininfo(1) and check out the "Depth:" line.

Today I suspect it's not used much, if at all. When you've got a 256MB video card, what's a few extra bytes? Modern graphics cards might not even support this.

But I believe that 15 or 20 years ago, this feature actually did get used. You'd have a depth=24 root window, and a depth=4 (say) hardware overlay for things like the mouse cursor and popup menus, because they didn't really need a lot of colors, and you don't want to redraw your expensive graphics every time you moved the mouse.

Of course, everybody thought that dealing with hardware overlays and custom colormaps everywhere was a royal pain for application developers (and it was), so we came up with higher-level toolkits like GTK+ which make it really easy to splat up a window without caring about this, and it'll just use depth=24 (or whatever you've got) everywhere, and we've got tons of video memory so nobody cares anyway.

I'm not saying X11 was particularly good at this, but it did support this particular feature.

It is a very cool feature, but can you think of a sensible use for it on a machine that can run the entire screen in high resolution and full colour?

Not anymore, but if there were no 64 bit machines...

Video ram mapping reduces available memory space. Clawing back a few tens of megs of system ram could potentially have been worthwile(1).

Supporting a 32 bit colour depth screen at 32,768x32,768 resolution would require 4gb of video ram, leaving 0 bits in the address pool for the rest of the system :)

(1)For very, very low values of worthwhile.