/etc/init.d directory exactly equivalent (functionally) to what I presume to be the more standard
/etc/rc.d/ (at least on arch)? Is there any particular reason canonical used init.d instead of rc.d for startup scripts?
/etc/init.d was the old historical location for SVR4. I forgot why redhat added the /etc/rc.d/ level. I think to isolate things onto rc.d, but then needed to add a bunch of symlinks anyway for backwards compatibility. So there is /etc/init.d in redhat, just it symlinks elsewhere.
So the standard location is /etc/init.d, though it may be a symlink not a real directory.
There were some really old Linux distros that copied BSD with /etc/rc.local but pretty much no one uses that anymore.