Some other deployment platforms such as Octopus Deploy have the concept of shared variables/properties/values across applications. For instance, there may be 25 applications that all consume an API at a configurable URL. In the case that the URL changes, it would be ideal to change that value in one place.
Is there anything in UCD that supports that type of arrangement?
While System level variables are global, the Resource tree variables are likely what you want. The Resource tree is a hierarchical, cascading set of variables that can be overridden at lower branches, but inherit from parent branches.
Environment in the Application
Here is the documentation on resources: https://www.ibm.com/support/knowledgecenter/en/SS4GSP_7.1.1/com.ibm.udeploy.doc/topics/getstart_resource_create.html