We wouldn't fault you if you haven't heard about macOS Server. The $20 app lived in the depths of Apple's software suite and allowed device management capabilities for people who want to govern their Apple hardware. The Cupertino tech giant barely talks about it and now, it has decided to finally discontinue macOS Server.
Apple's rationale for this move is that macOS Server's flagship features such as Caching Server, File Sharing Server, and Time Machine Server are already available in macOS High Sierra and later at no additional cost, and as such, there's little benefit in keeping its neglected product up to date.
Interestingly, Apple will continue selling the $20 app on its App Store for people using macOS Monterey. As such, the discontinuation only means that no new features will be added.
In terms of support, Profile Manager and Open Directory are supported in macOS Server 5.12 and later. Meanwhile, Xsan is also supported in macOS Server 5.7.1 up until 5.11.1, and command-line tools for Xsan management are already bundled in macOS Big Sur and later. You can find out the support status for all macOS Server services at Apple's dedicated webpage here.