Apple introduced iCloud Private Relay with the now released iOS 15 and upcoming macOS 12 Monterey.

Can the SimpleMDM Restrictions profile UI controls add the key to control that key before Monterey comes out? The feature was added to Monterey in beta 3.
(Beta 3) For supervised devices the use of iCloud Private Relay can be prevented with the allowCloudPrivateRelay restriction.

The key is allowCloudPrivateRelay documented at https://developer.apple.com/documentation/devicemanagement/restrictions