The Adafruit MPC23017 library is not compatible with Teensy4.

[rolfdegen]

The Adafruit MPC23017 library is not compatible with Teensy4.

I want to use an MPC23017 on the Wire1 port of Teensy4.1.
The library only works with Wire0. How can I set the MPC23017 on Wire1?
The I2C speed cannot be set to 400 kHz!

[caternuson]

The library only works with Wire0. How can I set the MPC23017 on Wire1?

Pass the address and non-default Wire reference when calling begin_I2C():
https://adafruit.github.io/Adafruit-MCP23017-Arduino-Library/html/class_adafruit___m_c_p23_x_x_x.html#adef0d2d16f9c2d558a3b72483d4f129b

The I2C speed cannot be set to 400 kHz!

That is not the responsibility of this library. It would need to be supported by the Teensy Arduino BSP's implementation of the Wire library:
https://docs.arduino.cc/language-reference/en/functions/communication/wire/setClock