(…) systemd is linux only. Ask why?

It is well known that systemd’s service management is built around cgroups, which is a Linux-only concept for now. Other OSs have their own ways to accomplish similar things, but adapting to that would require huge changes in systemd.

Am I able to run system-journal without any other systemd components running?

No, the only part of systemd project that doesn’t depend on systemd core is systemd-boot. And there’s also elogind, which is an independent project to lift systemd-logind out of systemd.

But honestly, I don’t see the issue here. You can’t use systemd’s components elsewhere, but your previous complaint was the opposite - that systemd is everywhere, as if you were forced to use networkd, resolved (which pretty much no distro uses AFAIK because it’s way worse than other DNS resolvers), homed, timedated etc. when you use systemd as init.

Also, I have a correction for my previous comment: systemd-journald is not an optional dependency, as it’s used as a fallback if the configured log daemon fails. I’ve only learned after writing that comment.