Were they alternate implementations of sysv-init? Or did they do their own thing, and also happen to run sysv-init scripts for back-compat? Because systemd also runs sysv-init scripts for back-compat.
Edit: Also, were any distros actually shipping those as supported init systems? I was under the impression that most of them were still in the "experimental" stage and not viable replacements (yet).
OpenRC and Solaris SMF definitely did their own thing. SystemV-init and the "run levels" concept was flawed from the very start when it replaced the previous BSD-style "single user" vs "multi user" boot up.
Linux only adopted SystemV style init because it was the norm in Solaris at the time. It is not a linux-ism.
As this article notes - systemd is actually fine as an init system and hardly anyone denies it. It's all of the other stuff (journald, resolved, timers, etc) + tight coupling + environmental assumptions that is the problem.
I'm unaware of any Linux distribution using SMF. If there was, I would have been running it. I didn't really like pfexec (it felt bolted on and not quite there yet), and I really really hated ZFS (and still do), but SMF made Solaris administration a joy.
I don't think any distribution was using runit before systemd, but it was available in Gentoo as a sysvinit replacement and ISTR it was used in the Rails community for supervising Unicorn.
Edit: Also, were any distros actually shipping those as supported init systems? I was under the impression that most of them were still in the "experimental" stage and not viable replacements (yet).