Most mesh devices (not wifi), i.e. thread/openthread devices that use 6LowPAN are IPv6 only. They're often based on microcontrollers with limited resources, so dont even support TCP, only UDP. Many smart meters also use this same set of technologies (IPv6/6LowPAN/UDP/CoAP/etc) which is my own area of expertise, but the Google (and Amazon) devices also use these.