Apple allows networks to block Private Relay:

"Network settings

Some organizations might be required to audit all network traffic by policy. To comply with such a requirement, these networks can block access to Private Relay. Users will be alerted that they need to either disable Private Relay for the network or choose another network. The fastest and most reliable way to do this is to return a negative answer from the network’s DNS resolver, preventing DNS resolution for the mask.icloud.com and mask-h2.icloud.com hostnames necessary for Private Relay traffic."

https://www.apple.com/privacy/docs/iCloud_Private_Relay_Over...