This is a "compilation" of a few web sources (see at bottom) of how to completely remove systemd from a Debian 8 system while retaining desktop functionality (with the likely exception of Gnome).
I did this to my LXDE Jessie system, and so far all works well.
** Only try this at your own risk! **
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First, install sysv-init:
apt-get install sysvinit-core sysvinit sysvinit-utils
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Reboot
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Purge systemd:
apt-get remove --purge --auto-remove systemd
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Prevent installation of systemd packages:
/bin/echo -e 'Package: *systemd*\nPin: origin ""\nPin-Priority: -1' > /etc/apt/preferences.d/systemd
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Add the "angband repo" to your repo lists:
/bin/echo -e 'deb http://angband.pl/debian nosystemd main\ndeb-src http://angband.pl/debian nosystemd main' > /etc/apt/sources.list.d/angband.list
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Save and add the repo's signing key from http://angband.pl/deb/archive.html
apt-key add <the saved keyfile>
apt-get update; apt-get dist-upgrade
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Search for any systemd "leftovers", and purge them:
dpkg --get-selections | grep systemd
apt-get remove --purge --auto-remove <remaining systemd packages>