Is the single zener to GND going to be enough protection?
All Arduino inputs are rated for +VCC+0.5 V = 5.5 V which is higher than a typical 5.1 V zener’s max of 5.1+5% = 5.355 V so should be safe, as long as you don’t fry the zener. Assuming 1/4 W components, the series resistor gives you enough headroom for 10 V and a bit over that.
(the inputs do have built-in clamping diodes too – see this post – but they can only handle a milliamp or so, so you’ll need a much larger resistor if you remove the zener).