@User How low you can go with VDDIO depends on the threshold voltage of the transistors. Higher threshold voltages means you need for higher VDDIO. Transistors that can support higher gate voltage typically also have higher threshold voltages. You can use the core (e.g. 1.8V) transistors to make an IO cell but then they will blow up when trying to use them at 3.3 or 5V. Also these transistor may not be designed to have good ESD performance so may need active ESD clamp.