This neatly drawn and carefully calculated schematic could have a problem: it assumes that the 24V AC sense supply will be exactly 24V when in practice that is only a nominal value that may not be exact and may vary. And, in the real world there is likely to be electrical noise on the signal and sometimes voltage transients or spikes that could be much greater than 24V. Those voltage variations and spikes will be passed on directly to the Vout 5V output, albeit at a reduced level.
The maximum voltage that can be safely applied to an Arduino input pin doesn't seem to be specified at a quick look, but unless the inputs are known to be protected on the board, it should be assumed that the input voltage shouldn't exceed around 5.5V or the board could be damaged.
The above design could be modified to eliminate the risk but it would mean adding two more components, a zener diode and a resistor, to clamp the Vout voltage at a level at or below 5V whatever the input. The R1, R2 potential divider would have to be redesigned to raise the voltage to 7V or more, exact value not critical and could be 12V, and then a resistor from that point would supply a back biased 4.7V or similar zener diode to provide a safe Vout voltage.
One of the opto coupler options above would probably be a simpler and safer solution.
I shouldn't really have said that voltage transients and spikes will be passed on directly to Vout because the quite large capacitor C1 will have a smoothing effect, but a zener diode or other means of clamping Vout at 5V or slightly below is needed to prevent the risk of damage to the Arduino board unless its inputs are known to be protected.