It is possible that we are witnessing the end of an era. MacOS Server has been available since 1999, and it is also known as Mac OS X Server and OS X Server in some circles. However, as noted by The Register, Apple deprecated several of the more server-oriented features in 2018.
All Mac OS High Sierra and later installations come with the most commonly used server features—such as Caching Server, File Sharing, and Time Machine Server—so that more customers have access to these essential services at no additional cost.
One of the primary reasons anyone would have continued to use macOS Server was Apple’s mobile device management (MDM) service, Profile Manager, which has now been deprecated. It has also published two support articles on the subject of other MDM solutions.)
It’s safe to assume that the app will continue to work with macOS Monterey, but administrators should start looking for alternatives now, even though Apple says “existing macOS Server customers can continue to download and use the app with macOS Monterey.”