Here is how i do it:
Fisrt you have install other init system other then systemd to replace systemd.
Here is how to install openrc:
sudo apt install openrc-init
Here is how to install runit:
sudo apt install runit-init
Next disable systemd package in apt for prevent apt install systemd
Put it in /etc/apt/preferences.d/systemd.pref to prevent apt install systemd.
Package: systemd
Pin: release a=*
Pin-Priority: -10
