"This is not available everywhere, not even on Linux distributions. Why would you implement something which only works on 50% of existing Linux distributions?"

According to http://0pointer.de/blog/projects/os-release systemd requires /etc/os-release. Since all major Linux distribution now use systemd, it makes sense to me to add such function.

We can document that the feature is specific to Linux (or don't declare the function on Linux?), document that it only works on recent versions of Linux distributions, and document that it doesn't work on all Linux distributions.