Following http://without-systemd.org/wiki/index.php/Debian_Stretch and adding a pin on *systemd* will lead to libvirt-daemon-system being uninstalled though (dependency chain: libvirt-daemon-system -> polkit -> libpam-systemd).

It is possible to build rebuild polkit without systemd, see https://github.com/bidaian/no-systemd, but that means tracking another package diversion.

An alternative solution is to just not use systemd as init, put keep the other packages installed:

apt-get install sysvinit-core sysvinit-utils
apt-get remove --purge systemd-sysv