systemd might be the dominant init system in the Linux world, but there are others that you'll still come across. At the time of writing, CentOS 6 is very much alive and kicking, with its upstart core.
That's without going into the nitty-gritty of what the BSDs use and macOS to boot (which I actually quite like).
I have seen some absolutely ancient Debian installations in my time, some of which went out of support half a decade or longer ago. I once worked out that a box I was SSH'd into, could legally drive.
