Pretty much all ICs these days have builtin ESD protection which usually involves current steering diodes to supply rails.

So if you limit the input current (typically <5mA, and assuming the steered-to rail uses at least 5mA to sink it) it will survive any voltage. (Of course if it's a low power design or the device is off this wont work that well, then you want external clamping)

I have tested 24V AC applied to an stm32f0 pin with a 10K series resistor and it survives that indefinitetly.