Your comments

Better yet: Forbid use of "AltGr" on PCs and "Alt" unless the key combination contains an additional Ctrl/Super/… key (‘real modifier’), or at least make an option to forbid such destructive behaviour.

For me, this is one of the most annoying reasons to switch to other editors repeatedly whenever I need ‘special’ (letters not used in postclassical English or classical Latin). Mapping to a non-existing command presupposes that (1) the key is a normal non-modifying letter (not the case with ‘dead’ keys in general or Ctrl-Shift-U on Linux) and (2) that users do not switch to a different keyboard for typing different languages or getting special letters.

If (1) does not hold, there is no way to get the desired behaviour even temporarily (reda ABDI's case).

If (2) does not hold, the key will work with one keyboard layout as expected, but will be broken when switching.