> What is the expected content in “out.txt”?

Who knows? The way that shell redirection produces files is entirely
outside the scope of the standard. std::print and std::print et al.
are simply stated to forward to vprint_unicode or vprint_nonunicode.

And again, this is not introducing any new problem, just perpetuating
an existing one. You have the same problem with std::wcout.

In any case, having more Unicode support would be useful in general.
If we add overloads for std::print that work with char8_t, char16_t,
and char32_t, then we may as well add the wchar_t overload. It's
fundamentally the same problem.