> Newlines are now forbidden in filenames

No. To quote that article

> A bunch of C functions are now encouraged to report EILSEQ if the last component of a pathname to a file they are to create contains a newline

This, yes, makes newlines in filenames effectively illegal on operating systems strictly conforming to the new POSIX standard. However, older systems will not be enforcing this and any operating system which exposes a syscall interface that does not require libc (such as Linux) is also not required to emit any errors. The only time even in the future that you should NOT worry about handling the newline case is on filesystems where it's is expressly forbidden, such as NTFS.