Sorry, I meant one implementation of the process that ran as PID 1. There are many implementations of the rc script part, as you point out.One init? Like openrc, sysVinit, upstart, runit?
All Linuxes I'm aware of used sysvinit before systemd. Ironically the attention created by the systemd thing spawned a bunch of new PID 1 implementations, including a compellingly simple one.