I think the OP meant s6 and not s9.

runit, s6 and also nosh are all daemontool-style init systems, which have a somewhat different idea how to do things than systemd all, but all are inspired by the original daemontools package.

s6 and nosh have decent overviews:

https://skarnet.org/software/s6/ https://jdebp.eu/Softwares/nosh/guide/introduction.html

The original daemontools: https://cr.yp.to/daemontools/faq/create.html#why

I'll let others decide whether these are better or worse than systemd. I go with whatever my distro would give me, since picking a different init system is too much work...