Mail, Messages, Calendar, Contacts, and Wiki.

These were fully integrated into the standard Sharing pane, removing the need for the Server app for basic network storage. The Survivors: Device Management

To streamline the OS, Apple moved several high-value features directly into the standard macOS client (High Sierra 10.13.4 and later):

Released in the spring of 2018, version 5.6 is most famous for hiding several core services from new installations. While users upgrading from older versions could still access them, fresh installs no longer saw the graphical interfaces for: