You can just uninstall it. It looks like apt remove systemd would work on my system, at the cost of GNOME and NetworkManager. APT automatically fills the gap with sysvinit and consolekit. OpenRC is also available.
Debian even has ports to kFreeBSD and HURD that don't support systemd at all.
There were some rough edges last time I ran Debian without systemd, and it might have gotten worse since then, but it surprises me how Debian ended up at the center of the debate when it doesn't seem that bad as systemd-defaulting distros go.