The arduino_rpc package includes functions to serialize/deserialize commands, where each command corresponds to a method signature extracted as described in the "Extract method signatures" section ...

🚦Example of using the 74HC165 Shift Register within the ESP8266 Arduino Environment. ... (hardware) interface. The hspi is far faster and should be the perfered method. But, where spi isn't available ...

That’s kinda mean. The Arduino already has a socket and a board. Just the ATMega328 is 5,65 euro. For 21 IO pins, that’s 0.26 euro per IO. The MCP23S08 is 1,32 euro, so 0.17 euro per IO.