Originally two separate services in Mac OS X Server, for Software Updates and later Content, it reached client versions of macOS in High Sierra in 2017.
Mac OS X Server
Introduced 20 years ago, it has just had its second major redesign and a huge boost in capability. Now used for cloud and colocation services, and widely as a server and desktop of choice.
A/UX was released for the Mac II in 1988, then in 1993 Apple changed course with a series of servers, before Mac OS X Server in 1999 and its first Xserves in 2002.
The Eclectic Light Company 