How to extend optocoupler's common mode transient immunity to its Input constant current driver
From the optocoupler's point of view, common-mode transients happen when the voltages at the inputs change relative to the voltages at the outputs, but when the voltage between the A and K inputs stays the same. In this circuit, this implies that the voltage between VDDiso and GNDiso does not change.
In other words: the LED driving circuit is not affected by common-mode noise.
If you want to protect your circuit against noise, you have to care about differental-mode noise (most likely due to ESD). Optocoupler LEDs have a very bad reverse voltage rating, so you should add an antiparallel diode. And a low-pass filter (as simple as a capacitor) will prevent the optocoupler falsely switching on or off due to short spikes.