TP-Link Kasa smart plug/bulb adapter for WebThings Gateway.
There's nothing technically preventing support for these devices, they just have not been tested at all.
TP-Link Tapo devices will NOT work, as they use an entirely different protocol.
If you're running this add-on outside of the official gateway image for the Raspberry Pi, i.e. you're running on a development machine, you'll need to do the following (adapt as necessary for non-Ubuntu/Debian):
sudo pip3 install git+https://github.com/WebThingsIO/gateway-addon-python.git