I2C lines are dark purple on the Teensy pinout card. 18,19,16,17,24,25 depending on which I2C peripheral you choose.
The MCP23017 chip on that shield will not interwork with 3.3V if powered from 5V - you'd need level shifters.
However there's a power jumper on the shield so you can disconnect the 5V from it, and presumably then power it at 3.3V
from the T4.1's 3V3 pin. The MCP23017 is compatible with 3.3V power. That gives 3.3V expanded GPIOs.