There's an argument to be made that system software like filesystems and kernels shouldn't get too smart about validating or transforming strings, because once you start caring about a strings meaning, you can no longer treat it as just a byte sequence and instead need to worry about all the complexities of Unicode code points. "Is this character printable" seems like a simple question but it really isn't.

Now if I were to develop a filesystem from scratch, would I go for the 80% solution of just banning the ASCII newline specifically? Honestly yes, I don't see a downside. But regardless of how much effort is put into it, there will always be edge cases – either filenames that break stuff, or filenames that aren't allowed even though they should be.