I was thinking that - but more than that I was thinking of FSs that allow UTF8 or UTF16 characters - which do not allow invalid code points. But from this morning's research that seems to only be NTFS and UTF16, which is not exactly a "unix" FS.

I don't think that's strictly true. Most unix filesystems "allow" UTF-8 characters, specifically because they treat the filename only as an array of bytes and don't interpret that array at all. Perhaps NTFS does do some work to present it as UTF-16 codepoints, I don't know, but it's far from the only file system that allows this to happen.

He may have meant accept only UTF-8/16. That is the very nice thing about UTF-8 though is that it plays so nice with routines that can accept ASCII or Latin-1. You can't use old routines to character count/change case/etc., but at least they won't corrupt your string by accident.