I labeled "good first issue" since I will take anything that passes a test :) Doesn't have to be complete.

I think it's basically passing through arguments to GNU readline, which is readline_mod if you grep the code, but I'm not sure.

Hope this link helps!
https://stackoverflow.com/questions/22224657/custom-readline-functions-in-bash-commandline

Hm that might help, but for the foreseeable future the interactive shell is off my personal TODO list. So I hope that someone makes a big effort in this direction, including understanding the bind builtin :)

The last section of the last blog post links to a comment that gives color on this:

http://www.oilshell.org/blog/2021/11/release-0.9.4.html#links

btw. the bash init script for atuin fails due to missing bind implementation (and also missing bash_versinfo, which is easily fixable):

bind -x '"\C-r": __atuin_history'
bind -x '"\e[A": __atuin_history --shell-up-key-binding'
bind -x '"\eOA": __atuin_history --shell-up-key-binding'