We use source /etc/os-release a lot across the repos to check which distro we are on for packaging, testing etc.
According to man os-release (visible at https://www.freedesktop.org/software/systemd/man/os-release.html):

The file /etc/os-release takes precedence over /usr/lib/os-release. Applications should check for the former, and exclusively use its data if it exists, and only fall back to /usr/lib/os-release if it is missing. Applications should not read data from both files at the same time. /usr/lib/os-release is the recommended place to store OS release information as part of vendor trees. /etc/os-release should be a relative symlink to /usr/lib/os-release, to provide compatibility with applications only looking at /etc. A relative symlink instead of an absolute symlink is necessary to avoid breaking the link in a chroot or initrd environment such as dracut.

Thus, I think we should try to source from /etc/os-release, and if that fails then we should try /usr/lib/os-release. Right now we only go to /etc.
This is not a problem for the distros we run on today, but may be in the future.

I wonder if:

source /etc/os-release || source /usr/lib/os-release

would work? If not, we might have to write a little helper func in the global shell lib.