With earlier versions of Calendar Server, Calendar Express (the old user interface) was always enabled, even if you did not use the interface. Now it is possible to disable Calendar Express explicitly, using the new ics.conf parameter, service.http.ui.enable.
If you are upgrading from an earlier version of Calendar Server, the upgrade process adds the parameter to the ics.conf file set to "y". This allows the legacy user interface to continue to be used without any changes. However, if you wish to disable it, set this parameter to "n".
Since Calendar Express was deprecated, and is no longer automatically installed in a fresh installation, the parameter does not appear in the ics.conf file. The default internal setting is "n".
If you intend to use Calendar Express in a fresh installation, you must install Calendar Express and then add service.http.ui.enable="y" to the ics.conf file.