Apple says that macOS Server — available at the Mac App Store for $19.99 — is changing to focus more on management of computers, devices, and storage on your network. As a result, some changes are coming in how Server works.
Designed for macOS and iOS devices, it’s 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.
Mac OS X Server was provided as the operating system for Apple’s Xserve computers, rack mounted server computers. At one point, it was optionally pre-installed on the Mac mini and Mac Pro and was sold separately for use on any Mac meeting its minimum requirements.
Soon a number of services will be deprecated, and will be hidden on new installations of an update to macOS Server coming in spring 2018. If you've already configured one of these services, you'll still be able to use it in the spring 2018 macOS Server update.
These deprecated services will be removed in a future release of macOS Server, so those depending on them should consider alternatives, including hosted services, says Apple. Deprecated services are listed here (https://support.apple.com/en-us/HT208312).