Apple announced today in its support document that macOS Server support 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 close 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 moved to macOS. Popular macOS Server features, including the caching server, file sharing server, and Time Machine server, have been merged into macOS starting with 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 application from macOS Monterey.