This is rarely given by any manufacturer, I see it asked quite a lot in Microchip forums.
If I understand it the ESD is actually a byproduct of using a PMOS high switch but I could have that wrong. Roughly they start conducting at 0.3V but that still doesn't mean you can go above Vdd on the pins. Generally we're talking uA through it. You might get a mA or even a few but for how long nobody knows as ambient temperature and manufacturing tolerances come into play.
The best advice I've found is a) don't use the 'feature' unless you have to and b) if you do use it on occasion then go large on the resistance.
As an example I have a 32V AC signal and I capture it using GPIOTE using a 732k/732k divider. That's 16V through each one, Vdd is 3.6V, so 12.4V/732k = 1.69mA peak. It works and has done long term but even this I think is high and the next PCB revision will go to 1Meg.
"Maximum ratings are the extreme limits to which the chip can be exposed for a limited amount of time without permanently damaging it. Exposure to absolute maximum ratings for prolonged periods of time may affect the reliability of the device."
There are ESD diodes that can handle short transients/spikes if voltage exceed +-0.3V off VDD/GND, but they are not designed for continous current.
It's mostly the same thing up to the point at which it blows. The difference between zener (usually thought of as a clamp as it has a tighter knee making it more suitable for regulation) and esd/tvs is generally one of capacitance, which determines how fast acting it is.
