the byte 0x5C occurs as second byte of some multibyte characters. If such a
character is used inside a directory name, code that uses ISSLASH does not
work correctly. All gnulib modules that use ISSLASH are affected.
Could this also be a problem on Unix systems using multibyte encoded
(UTF-8) filesystems, if not now then in the future?
Nope. Unix kernels/filesystems don't care at all what encoding the
file names are in. Encodings are handled in userspace. The only thing
that matters is that a '/' (0x2F) or '\0' byte can't be part of a
directory entry name. I don't think this is going to change.
Maybe some (future) Unix systems support multi-byte encoded filenames
containing 0x3F in the second+ byte of a multi-byte character.
0x3F is '?'. You mean '/', 0x2F? I very much doubt that.