macOS Server discontinues calendar and contacts services.
https://support.apple.com/en-us/HT208312
As hosted services have become widespread—and the need to keep software up to date is more important than ever—Apple is making some changes to macOS Server.
Apple continues to develop and support macOS Server, which includes Open Directory, Profile Manager, and Xsan management. The most popular server features—Caching Server, File Sharing Server, and Time Machine Server—have been bundled with every installation of macOS High Sierra since its release in fall 2017, so that even more customers have access to these essential services at no extra cost.
Starting with macOS Server 5.7.1, Apple no longer bundles open source services such as Calendar Server, Contacts Server, the Mail Server, DNS, DHCP, VPN Server, and Websites with macOS Server. Customers can get these same services directly from open-source providers. This way, macOS Server customers can install the most secure and up-to-date services as soon as they’re available.
Apple has published documentation to help with this transition.
https://help.apple.com/serverapp/mac/5.6/#/apdAC95F5C4-27AA-4378-8BB9-95A79A64AB5D