Our /cc APIs have never been formally documented as they were meant to be private and used only by the UI. You might find parts and pieces of the /cc API documented here and there, but there is no official, central location for these docs. We are hard at work providing v3 functionality for our older APIs. However, if there is functionality in the UI that you are trying to leverage in your code, and you can’t find an equivalent v3 endpoint in our docs, your best bet is to reverse engineer the UI by using your browser’s inspector tool.
It depends on what you are trying to do with the /user endpoint. If you need to search for accounts or identities, then you can leverage our search API. @darrenjrobinson has created a PowerShell module for IdentityNow APIs. You can see how he implemented list users.