Again I know this is an old topic but I found it when trying to resolve this myself.
My understanding is that /System/Library/StartupItems has been deprecated - I haven’t actually tested that. Apple’s approach is to use a plist file in /Library/LaunchDaemons
Again as I understand it this is more for developers but I have managed to make it work.
See this link:
https://community.roonlabs.com/t/start-roonserver-on-a-mac-at-startup-not-login/94641?u=marcus_russell