Does a bit set in a port's OVTDIS register disconnect the corresponing I/O pin's clamping diodes from the supply rail? If so, does that affect the absolute maximum input voltage rating for the pin?
The answer is yes in both cases.
On EFM32 Series 1 and EFR32 devices, any GPIO pin for which the corresponding OVTDIS bit is set should be considered to have an absolute maximum input voltage of IOVDD + 0.3V.
Raising the pin above this voltage will drastically increase its leakage current and is not safe for long-term reliable operation.