Magnetometer.z

The z read-only property of the Magnetometer interface returns a double-precision integer containing the magnetic field around the device's z axis.

If a feature policy blocks use of a feature it is because your code is inconsistent with the policies set on your server. This is not something that would ever be shown to a user. The Feature-Policy HTTP header article contains implementation instructions.

let magSensor = new Magnetometer({frequency: 60});

magSensor.addEventListener('reading', e => {
  console.log("Magnetic field along the X-axis " + magSensor.x);
  console.log("Magnetic field along the Y-axis " + magSensor.y);
  console.log("Magnetic field along the Z-axis " + magSensor.z);
});
magSensor.start();