The Sonoff S20 has only a 4 pin header (GND, TX, RX, VCC).

To use this pins (TX=GPIO1, RX=GPIO3) for sensors, the serial interface has to be disabled:
Tools – Advanced – Enable Serial port: uncheck

Now it is possible to use gpio1 and gpio3 for Devices:

This is the first test on a breadboard with DHT11

I don’t think that a humidity sensor makes any sense inside the socket. Therefore, I think I will use DS12B20 soldered on a female pin header in the final version.

And this is the result:

The values are also published via mqtt:

# mosquitto_sub -h 192.168.234.100 -t "#" -v
/sonoff-s20/DHT11/Temperature 24.00
/sonoff-s20/DHT11/Humidity 33.00