Apple today announced in its support document that support for macOS Server ended on April 21, 2022. That is yesterday.
Apple has been phasing out macOS Server for several years now, and now the company is ready to shut down the project for good.
macOS Server 5.12.2 will be the latest version of the operating system, and macOS Server services have now been ported to macOS. Popular features of macOS Server, including the cache server, file sharing server, and Time Machine server, have been merged into macOS since macOS High Sierra, so there is no longer a need for a separate version of the system.
Existing macOS Server users can continue to download and use the app with macOS Monterey.