The os-release file is not tight to systemd. Only the reverse relationship is true: systemd, d-bus and other software require os-release.

The file is present in the minimal base image of distributions like Alpine, ArchLinux, CentOS, Debian, Fedora, RHEL, SUSE, and Ubuntu plus all derivates. In the last three years I have not seen any Linux distribution that is missing the file. The only odd-ball in the list is Alpine. It ships /usr/lib/os-release without the optional but recommended /etc/os-release symlink.

If you know any Linux platform without os-release, please let me know. I'm curious to know which platforms don't have the file and why the platform excludes it.

Python could also follow the lead of other software like D-Bus and make the presence of os-release mandatory on Linux. Any Linux platform without it would be considered broken. I don't think it is necessary to impose such restriction on vendors.