/etc directory. /etc include *-release files, depending on the distribution, include alpine-release
arch-release
debian_version
gentoo-release
oracle-release
redhat-release (which I found to co-exist with oracle-release in a WSL Oracle 9.1 distribution).
SuSE-release
system-release (which I found to be a symlink to oracle-release in the aforementioned WSL distribution)
os-release.