I'd assume that the proper place for defining what's a valid filename would be on the filesystem level, so a filesystem of standard ABC v123 would not allow non-unicode names; so non-unicode filenames would either get refused or modified upon copying/writing them to the filesystem.

This is not new, this would match the current behavior of the OS/filesystem enforcing other character restrictions such as when writing (for example) a file name with an asterisk or colon to a FAT32 USB flash drive.