https://wiki.gentoo.org/wiki/Gentoo_Without_systemd
Looks like you have to do a bit of tinkering to ensure it stays free of systemd. Same with Debian, which I use with SysV init. But it took a couple of hours tweaking to switch - perhaps one of the other inits might have been easier to switch to.
I've never tried to move debian to not-systemd.
https://wiki.gentoo.org/wiki/Gentoo_Without_systemd
Looks like you have to do a bit of tinkering to ensure it stays free of systemd. Same with Debian, which I use with SysV init. But it took a couple of hours tweaking to switch - perhaps one of the other inits might have been easier to switch to.