Hey
thanks for this very useful library! In an existing hardware project I needed to replace a MCP23017 chip by its SPI counterpart MCP23S17. Although I could not find a library for that chip with the same software interface, I decided to take your library and modify it for the SPI interface. So could save time, not adapting existing software. (besides class instantiation)

The result can be found here https://github.com/felix1024/MCP23S17

tested with Arduino framework on ESP32 using platformio.

Hopefully this OK for you.