>> We cannot draw a conclusion from a single example, and AFAIU the 
>> screenshot you show does not tell that "x" is used to maximize a window 
>> but only that, when that menu is opened, you can select its "maximize" 
>> entry by pressing "x".
>
> How many examples do you want? Windows, KDE, Xfce, pretty much every one 
> with the concept of a window operations menu.
>
> Yes, those are menu item mnemonics, not direct bindings, because no 
> window manager is going to steal character input for window operations. 
> You typically invoke the menu with Alt+Space, then use the mnemonic; in 
> this use, it is similar to Emacs’s prefix maps.
>

I don't use any of these window managers myself, which is why I asked for 
examples.  I misunderstood his answer, and did not realize that users 
could use a window menu the way you describe it.

>
> Alternatively, the CUA standard used to specify Alt+F10 for maximization 
> and Alt+F9 for minimization; Xfce still honors that in default 
> keybindings.
>

These are the kind of key bindings I had in mind, and a few Google 
searches showed a great variety of bindings accross window managers.

Anyway, it seems to me that this is tangent to the discussion.  The fact 
that some users use M-SPC x (in Emacs key notation) to maximize a frame 
(in Emacs parlance) does not seem to be a good reason to use the 'x' key 
to toggle the maximization of windows in Emacs.  At least I'll continue to 
use the 'm' key in my configuration for that purpose.