Saved from without-systemd.org and it's Stretch update (updated and simplified for bullseye by myself)
These instructions are for installing the sysvinit init program on Debian systems. This is done by installing the sysvinit-core package for an init system that's not systemd, and a set of elogind packages to provide backwards compatibility with the systemd library packages.
To install the sysvinit-core package and remove the systemd packages, run:
apt install sysvinit-core
reboot
echo -e 'Package: systemd-sysv\nPin: release *\nPin-Priority: -1\n' > /etc/apt/preferences.d/nosystemd
apt remove --purge --auto-remove systemd