How to remove systemd from a Debian jessie/sid installation
First install the SysV init packages
# apt-get install sysvinit-core sysvinit sysvinit-utils
Then reboot your machine and remove all of the systemd packages. BE AWARE that the following command removes packages that depend on systemd itself or things like libpam-systemd!
# apt-get remove --purge --auto-remove systemd
Prevent apt from installing systemd packages in the future.
# echo -e 'Package: systemd\nPin: origin ""\nPin-Priority: -1' > /etc/apt/preferences.d/systemd
Prevent apt from installing any systemd like packages in the future (note the star before and after systemd)
# echo -e '\n\nPackage: *systemd*\nPin: origin ""\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd
Note: This prevents the installation of the package systemd-shim as well. However this is a emulation of systemd functions, so you can install software which depends on systemd. So if you get "Package systemd-shim is not available, but is referred to by another package." you might temporary disable the three lines in the preferences.d/systemd file.
In the case your system uses multiarch (mixed 32 and 64bit packages), you need to pin the 64bit version of systemd too. Otherwise apt-get could still install it as dependency later.
# echo -e '\nPackage: systemd:amd64\nPin: origin ""\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd
In other multiarch cases where amd64 is the default architecture, you may have to pin the i386 package to prevent APT from installing it:
# echo -e '\nPackage: systemd:i386\nPin: origin ""\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd