If you’re looking at file system in the purist possible sense, you’d be right. But equally a lot of other meta data is stored beyond the inode that file systems

If you’re looking at file system in the purist possible sense, you’d be right. But equally a lot of other meta data is stored beyond the inode that file systems “understand”. And there is already precedence of file systems having code to intelligently handle file names (eg options for case sensitivity). So pragmatically it’s not an unreasonable suggestion to house any code defining legal file names in the file system driver too. The biggest argument against that in my view isn’t down to testing but DRY methodologies: if the code sits in the kernel then it should work against all file systems and not just supported ones.