Comment 4 for bug 1983399

> This is the first I've heard of any advice to avoid LC_ALL, and I wonder what is prompting that advice.

I didn't dig deep into the reasons, but there's plenty of authoritative resources in the internet who discourage using it.
See for example: https://wiki.debian.org/Locale

> It would be a pain to set every other LC_* environment variable to en_US.utf8

Well, you don't have to do it, because you can use LANG. And if you don't care about the sessions created by the console logins (most users nowadays just login from the graphical session), most desktop environments offer a configuration option in the user settings, where you can set the user's locale. Alternatively, the link from Debian I posted above contains a few other options.