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No, requesting a locale that doesn't exist doesn't error out, because we don't check the return code - it just keeps working the same way it does now (i.e. falling back to the legacy C locale).
However, it would be entirely reasonable to put together a competing PEP proposing to eliminate the reliance on the problematic libc APIs, and instead use locale independent replacements. I'm simply not offering to implement or champion such a PEP myself, as I think ignoring the locale settings rather than coercing them to something more sensible will break integration with C/C++ GUI toolkits like Tcl/Tk, Gtk, and Qt, and it's reasonable for us to expect OS providers to offer at least one of C.UTF-8 or en_US.UTF-8 (see https://github.com/python/peps/issues/171 for more on that).