I looked into this a little bit. Alternatives include OpenRC (complements but does not replace init), runit, upstart (looks like most distributions that used upstart are switching to systemd), and GNU Shepherd.

I am looking at trying out GNU Shepherd as it is the init system of the Guix distribution, so you get both Nix-style package management and an init system that is not systemd, both written and configured in Guile Scheme.

https://www.gnu.org/software/shepherd/ https://www.gnu.org/software/guix/