This is not a request for the current model of the phone, but something for a future model…
On the back of the phone have 5 sliders. When moved to the off position they would break the connection for power between the phone and
For those times when you absolutely positively want to be safe, remove the power from the connections to the outside world, and in the last case, physically disconnect the battery from everything.
I can think of a certain annual gathering in Las Vegas where any sane person would feel much better being able to push those switches. Just because you are paranoid does not mean someone is not out to get you.