The current functionality: REST services directory is either on (publicly available) or off (disabled) in an ArcGIS Enterprise setup with federated ArcGIS Servers.
The problem: We are in an organization where we have to disable access to any resources that do not require authentication. This means shutting off the REST services directory(ies) in an ArcGIS Enterprise. However we have developers and data scientists who would benefit from access to this directory.
The solution: place the REST services directory(ies) that are part of an Enterprise behind the same authentication regime set in the Portal.
If this is already possible to configure, please let me know - as I am operating under the assumption that this is not currently possible.