Most utilities that create files are encouraged to error on newline filenames, which makes this effective illegality stronger. The post also discusses the future of this encouragement, which is turning it into a requirement.

> However, older systems will not be enforcing this

Eventually, newlines in filenames will go the way of /usr/xpg4/bin/sh.

I'd like to note that up until this point, there hasn't (and isn't) been a fully POSIX compliant way to do many shell operations on newline containing filenames. They are already effectively unsupported, and the standard that adds support also discourages them from being created and used. The best way to handle them up until this point has been to not use sh(1).