> LC_CTYPE=C PYTHONCOERCECLOCALE=0 python3 -E ...

The UTF-8 Mode has the same behavior: PYTHONUTF8 env var is ignored when using -E, but enabled by the C locale: you can use -X utf8=0 in that case to ensure that the UTF-8 Mode is disabled.

I wanted to propose you to add a new -X option, ex: -X coerce_c_locale=0, which would have the priority over the environment variable. I'm not a big fan of env vars, it's not always convenient to use them. I prefer an env var, but both are even better :-)