While in the process of migrating some servers from my testing network, a few of them installed systemd; I was a bit surprised as I am using sysV, and have systemd pinned to -1.What happened? 
systemd apt pinned to -1 and installed in upgrade from Debian 8 to Debian 9
debian;apt;dist upgrade
It seems my systemd pinning to -1 was not enough. There are a couple more packages belonging to system to APT pin too.So, I changed my /etc/apt/preferences.d/01nosystemd to:Package: systemdPin: origin Pin-Priority: -1Package: dh-systemdPin: origin Pin-Priority: -1Package: systemd-shimPin: origin Pin-Priority: -1Package: libpam-systemdPin: origin Pin-Priority: -1And also ran the commands:sudo apt-mark hold systemd dh-systemd systemd-shim libpam-systemdAdditional note: libsystemd0 is not forgotten, simply it cannot be avoided so easily, due to several dependencies.Link: linux.org wiki questions - Prevent systemd installation