https://github.com/jankais3r/Siri-NoLoggingPLS
Disable server-side logging of Siri requests for your Mac, iPhone and iPad
<key>allowAssistant</key><false/>
Also note that you don't need a supervised iOS device for it, it can be installed on any Apple device.
https://github.com/jankais3r/Siri-NoLoggingPLS
You can disable Siri (and Apple Intelligence) entirely via Apple Configurator or asking the nearest LLM for .mobileconfig file with: