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All you have to hold is the power button to disable Touch ID. Adding the volume button does nothing additional on my iPhone 8.

You have to do power + volume down to get to that screen if you have a phone with no Touch ID (iPhone X and above). Just confirmed on mine.

It should work on iPhone 8. Perhaps you need to upgrade your iOS?

Pressing both should go into a mode where you can swipe to call emergency services or show your medical id if you have the feature.