[Reelin]

Neither have I outside of photographs! The trouble is that they did exist previously and X11 settled on an abstracted model that supports 5 modifiers (in addition to control, shift, and lock). You have to bear in mind that Emacs can't just stop supporting certain (now defunct to the mainstream) modifiers as many users have setups which depend on them.

I suppose that cosmetically they could update the documentation by changing M- to A- or Alt- or something. Would it really make a difference though?

Aside: Not meaning to be pedantic, but at least under X11 Super is the "Windows Key" and Meta doesn't exist by default. I just checked and (on my machine) the keycap with the Windows logo maps to X11 keycode 133 (hardware specific) which produces keysym Super_L at both levels 1 and 3 which in turn maps to mod4.