Custom XML Samples can be found here:

• https://github.com/vmware-samples/euc-samples/tree/master/iOS-Samples
• https://github.com/vmware-samples/euc-samples/tree/master/macOS-Samples/CustomXMLProfiles/Fall-2021

The attached status 400 is being returned in Google Chrome on macOS.

Will be a nice GUI in UEM?

Am I correct to say at this time there's no option to manage 'Mail Privacy Protection' on iOS 15 through any payload? 

@chengtmskcc - You are correct.   Apple has only provided a restriction to disable DNS Private Relay, but nothing yet for toggling Mail Privacy Protection: https://support.apple.com/en-us/HT212791