I guess many of the systemd detractors don't even realize that /etc/os-release is a systemd-ism.

Over the years the failed acquired various fields. It no longer just describes the distribution a system was installed with, but on image based systems it describes the image itself. It gained fields for describing the end of support, and where to find the release notes. And it's extensible: various distributions add their own fileds (hopefully properly prefixed by a namespace prefix!).