From how to disable X server autostart

The disable didn’t work because the Debian /etc/X11/default-display-manager logic is winding up overriding it.

In order to make text boot the default under systemd (regardless of which distro, really):

systemctl set-default multi-user.target

To change back to booting to the GUI,

systemctl set-default graphical.target

I confirmed those work on my Jessie and Stretch.