For prose of course it wouldn't be acceptable if you can't use your full language, but this is a filesystem, and we're talking identifiers, not prose. We accept that for a wide range of everyday things — I don't think many people are getting worked up about web forms not accepting their phone numbers when entered in Latin numerals.
Using bytestring labels sans normalization is just the rational choice, and actually quite accommodating. People _can_ use their natural language. If you can encode your prose in bytes, then there you go, that'll be your filename ;-) and if you can't remember what diacritic-combining strategy you used when you created the file, and can't manage to fopen() it anymore by typing out the filename, use a file picker or search. And then, perhaps, go "that was a hassle, I'll avoid those fu͝n͜n͏y͝ sq̧u͜igg̶l̢įȩş for my next filename, computer are stupid" - not the end of the world, not the worst outcome.
Then there is Vietnamese which has a lot of accents and other diacritics. Not sure how much you can do without them.
Perhaps a language like Chinese doesn't have so many ambiguities, as in there is only one way to encode a given character in Unicode?
The unambiguously encoded filenames wouldn't be in the natural language of the user, so you're asking the user to learn another language. Users shouldn't have to learn another language to use the computer.