Will Sonoff S26 Zigbee plug work with HA SkyConnect out of the box?

Sorry, that’s just wrong advice.

Most zigbee devices WILL need a quirk. @scotrod Have a look here if you’re using ZHA, or here if you want a more detailed list if you’re using Z2MQTT.

What’s the device that

?

Edit: This summarizes the state of zigbee at the moment