How do I keep Siri from disclaiming that she may not work without wifi

Siri needs an internet connection to do anything other than simple commands for built in Apps (like the Music app). So the warning is there to alert you she may be unable to respond with WiFi off. It’s a built in warning and there is no setting to disable it.

You can't, that's because if you don't have cellular, siri is basically killing herself and unable to turn WiFi back on.