As far as i know, systemd is creeping in as a dependency of a lot of unrelated packages - even UI packages. So you can have a Debian that doesn't run systemd, perhaps, but not a Debian that doesn't depend on bits of pieces of the systemd OS any more.
You have a tiny systemd library which does little more than checking if systemd is running. It's a helper library for doing that check. Within Devuan they didn't want this dependency, but that's unrelated from being able to use software under another init system.
There's a huge difference between not accepting any dependency with systemd in the name vs being able to use another init system.
That some software has a dependency does NOT mean that the software is restricted to only run under systemd.
the entire point of devuan (iiuc) is to have a clean separation of function. if systemd is a wide dependency, then the separation of the init system is a bit superficial and isn't what the distro cares about.
But these are hard deps not just recommends so will pull in systemd dependencies regardless of it using them or not. If it were a recomends type dep, again no one would have a problem. It used to be considered bad packaging to hard dep when recommends or suggests would suffice.
7.2 states: "When selecting which level of dependency to use you should consider how important the depended-on package is to the functionality of the one declaring the dependency"
You say "being able to use software under another init system." Looks like the level of dependency should be adjusted to fit your point.
You might want to learn how shared libraries work under Linux. Then you might be able to understand why packages have a Depends: libsystemd0 (which btw. doesn't depend on systemd itself).
If you want init system choice, run Debian (or e.g. Gentoo). Devuan seems more about hating systemd than any actual choice.