Don't know if they've changed it since OpenBSD 5.2, but /etc is very critical there.

E.g. man page synopsys for just two programs:

   /etc/rc - command scripts for system startup
   /etc/netstart - command script for network startup

There are countless others there. /etc/daily, /etc/weekly, etc (pun intended).

SunOS certainly did, Solaris does, AIX has at various points... probably the majority of Unix variants, in fact.

Linux, /etc/init.d/ or /etc/rc.d/init.d/