>> In frame.el's toggle-frame-fullscreen and toggle-frame-maximized, these look like either typos, or vestigial
 >> references.  The frame-parameter fullscreen is never set to 'fullscreen nor is the value 'fullscreen used in
 >> other places in Emacs.  It is an undocumented value, appearing nowhere.  Unless I missed something.
[---]
 > Martin, any comments?

I can only speculate.  Jan changed the semantics in 2009 so the previous
'fullscreen' became 'maximized' or 'fullboth' - I don't remember which.
For compatibility sake, he kept 'fullscreen' around since people might
have had (and maybe still have) it in their init files.  It's no more
documented to avoid that people start using it.

In either case 'fullscreen' still is a valid substitute for 'fullboth'.
Try with

(set-frame-parameter nil 'fullscreen 'fullscreen)

martin