mask_C1 is now a write-only variable. What does vis() do with C1 control characters?

Is the cast to size_t actually required?

Don't we need to limit that parameter to min(MB_LEN_MAX, <# bytes left>)?

When can mbrtoc32() return a value greater than n? It's not clear to me from reading the man page that that's permitted.

This might as well be a memcpy(). strncpy() doesn't guarantee nul-termination here, the assignment after the loop does.

From my testing, vis() automatically converts C1 characters. I added back the old behaviour for when mask_C1 is false.

end_q - q measures the number of bytes remaining in the output buffer, but we're passing the input buffer to mbrtoc32(). In particular, mbrtoc32() assumes that it can access up to n bytes of the input buffer, which is not always true.

max_len should be of type size_t.

It's better to write end_p = p + strlen(p) and check end_p - p rather than recomputing the entire string's length for each wide character.

Just setlocale(LC_CTYPE, "") probably.

Only three locales are defined by default, the empty string "" which
denotes the native environment, and the "C" and "POSIX" locales, which
denote the C language environment.  A locale argument of NULL causes
setlocale() to return the current locale.

If LC_CTYPE is unset getenv returns null, which causes setlocale() to just return the current setting. The empty string sets it according to environment variables automatically.