Passcode to Power Off iPhone 16 Pro Max

ios devices never supported a passcode to turn off or restart

judging by how many people experience issues which requires them to restart their device then they would be in big trouble if that would require a passcode they might have forgotten.

but you can suggest apple implement the feature by using their feedback channel

Product Feedback - Apple

Many people who ask for this feature say that they want it so that a thief cannot avoid tracking by shutting off a stolen iPhone.

The problem with that line of reasoning is that thieves often put stolen iPhones into a special type of bag which blocks radio transmissions. Even if the thief could not shut off the phone, the thief could block tracking. By the time the thief took the phone out of the bag, they might be far away and long gone from the scene of the crime.

Meanwhile, the feature would leave a legitimate owner in a real bind if they had to restart or reset their phone and they had forgotten the passcode.