Xserve had LOM hardware 20 years ago.
LOM enables power management even if the Xserve is off, and even if it lacks an installed operating system.
https://en.wikipedia.org/wiki/Server_MonitorEdit: Xserve was an Apple rack mounted server that ran a special version of Mac OS X
This isn’t quite lights-out, and Xserve ran the same version of Mac OS X Server you could install on any other Mac.