Hello Gilles,
Typically the NFC antenna is not touchable. Thinking about access reader or EMVCo payment terminals, the NFC antenna is several mm below the surface, covered by plastic or any other material and not touchable. Thus an electrostatic discharge is quite unlikely.
Nevertheless there are four pins indirectly connected to the antenna:
RFI: There is the capacitive voltage divider which has a ration of typically 1:12.
RFO: The RFO pin is connected through the matching network. At 13.56MHz it at least has a similar ration (1:10) as the RFI pins. Since it has a 13.56MHz bandpass characteristic, it will also reduce the energy of the ESD pulse.
The ESD rating per pin of the ST25R3916B is 2000V HBM (DS13541 Rev 9) - which is a common value for our devices. Please check the datasheet for specific devices / values.
If you still want to apply an ESD protection to the antenna, feel free to do so. There is no reason against it.
Just be aware that the ESD Diode has a certain capacitance, which need to be accounted when calculating the matching network.
BR Travis
