This should be an RFC.
How hard would it be to turn this into one?
There is already a de facto standard for extended codes: Xterm's modifyOtherKeys.
Applications:
https://vimhelp.org/map.txt.html#modifyOtherKeys
https://github.com/emacs-mirror/emacs/blob/d68f2b8681f8eeb6b...
https://github.com/akinomyoga/ble.sh
https://github.com/mawww/kakoune/commit/47c0d2038807757207cd...
https://github.com/tmux/tmux/wiki/Modifier-Keys#extended-key...
https://github.com/magiblot/tvision/commit/a50569f549465c9ed...
Terminals:
xterm (obviously)
https://github.com/gnachman/iTerm2/commit/ac1aa575011028ed86...
https://github.com/kovidgoyal/kitty/commit/d360d077d1f7723b1...
https://github.com/tomszilagyi/zutty
https://github.com/mintty/mintty/blob/master/wiki/CtrlSeqs.m...