Apple has announced that, as of April 21 (that’s today) it’s discontinued macOS Server.
Most of its features have been integrated into macOS proper. Apple says existing macOS Server customers can continue to download and use the app with macOS Monterey.
Designed for macOS and iOS devices, macOS Server is (was?) designed to make it easy to share files, schedule meetings, sync contacts, develop software, host a website, publish wikis, configure Apple devices, remotely access your network, and more.