Isn't x^2 typed <x><^><space><2>, while x² is the same minus the <space>, on most keyboards?
Recently I used Windows 10 quite a bit, and the number of AltGr combinations available on Linux has been the thing I missed the most!
It might work with a US keyboard as long as your computer was configured for an Italian keyboard, though. https://en.wikipedia.org/wiki/AltGr_key suggests that from the keyboard's perspective, AltGr and right Alt are the same thing. Really, no keyboard allows any character -- they just send key codes (and modifier codes), and it's on your software to interpret those. But typing ² is not part of the normal, expected functioning of a US keyboard.
Edit: it occurs to me that perhaps I should gloss "it's on your software to interpret those" as "it is the responsibility of your software to interpret those".
Just for testing, I installed a few new keyboards on my KDE desktop, and I can confirm that French, German, Spanish, and Greek keyboards have support for AltGr combinations to get superscripts/fractions/other characters.
I've never required that a keyboard could allow any character. However, this is indeed possible today, as Shift+Ctrl+u+HexNumber allows to quickly insert any Unicode character. (This works under Xfce and KDE, and probably other DEs.)
Nevertheless, I think that a 100-and-more-keys keyboard should support a subset of the most useful ones. I am a physicist and an Italian, so having quick keyboard combinations for exponents, fractions, and the euro sign € is extremely handy for my everyday activities.
The other abominations on those keyboards are the shorter left Shift key and the vertical Enter, both of which put two of the most used keys one position further away from home row.