systemd vs s6 Comments
systemd is not portable, is not truly modular and is not small code-wise.

- systemd is Free and Open Source
s6 is described as 'Small suite of programs for UNIX, designed for process supervision. It can be used as an init system. You can also use all its programs separately' and is an app in the os & utilities category. There are more than 10 alternatives to s6 for a variety of platforms, including Linux, BSD, Mac, FreeBSD and Docker apps. The best s6 alternative is systemd, which is both free and Open Source. Other great apps like s6 are OpenRC, runit, Launchd and sysvinit.
systemd is not portable, is not truly modular and is not small code-wise.

runit is a cross-platform Unix init scheme with service supervision, a replacement for sysvinit, and other init schemes. It runs on GNU/Linux, *BSD, MacOSX, Solaris, and can easily be adapted to other Unix operating systems.
a free system array created by Apple inc. For BSD - including macOS as macOS is based on the Darwin kernel of BSD
The Sysvinit package contains programs for controlling the startup, running, and shutdown of the system.
hummingbird is an init system designed for speed. It does nothing more than start the system and stop the system by default. There are three main "runlevels", fs which is called directly after the pseudo filesystems necessary for the system to run have been mounted...
People who use s6 are precisely the ones who want to go away from systemd.