I don't know if there is an explicit mandate for that in the standard, but forbidding this that were previously allowed, both in code and documentation, is not

I don't know if there is an explicit mandate for that in the standard, but forbidding this that were previously allowed, both in code and documentation, is not backwards compatible. Just imagine a file system that wouldn't allow the character "e" in file names. Of course, the impact would be not as drastic, but still it's backwards incompatible.