My goal was to bind Cmd-Backspace (labeled "delete" on a US Macbook Pro) to backward-kill-line. With fish 4.0, fish_key_reader, which displays friendly key sequence names, I was initially surprised that my terminal would not pass through Cmd. But -V showed what the problem was:

# Just backspace works as expected.
$ fish_key_reader -V
Press a key:
# decoded from: \x7f
bind backspace 'do something'

# Cmd-backspace sends the right escape sequence, but lacks a friendly name.
$ fish_key_reader -V
Press a key:
# decoded from: \e\[127::8\;9u
bind backspace 'do something'

If it's an easy fix, I can contribute a change... if someone shows me where.

In case it matters, I'm using Wezterm on macOS with the kitty keyboard protocol enabled (to pass SUPER aka CMD).