Holdouts can be enabled by selecting holdout geometry and clicking the Holdout button in the RenderMan shelf.
This has two effects. It enables the selected shape's Hold-Out attribute:
It also enables the Output Shadow AOV option under the Advanced render settings:
Holdouts can be visualized in Maya using image planes with the Image Plane As Sample Filter Advanced render setting enabled. This setting is enabled by default.
To turn off the Holdout AOVs (prevent output) you can deselect the "Output Shadow AOV" as noted above.