Hi Brad
My understanding is the following: Espressif (the makers of ESP8266) provides two different SDK's - one is based on FreeRTOS and one isn't (non-OS SDK). The non-OS does not support IPv6 (even due to the fact that it's based on lwip, which supports IPv6). The ESP8266 Arduino uses the non-OS SDK, hence it doesn't support IPv6. The RT-SDK should support IPv6 but as long as the SDK used by Arduino does not support IPv6, there's no way that the Tasmota firmware will support IPv6
As for the ESP32, only a RT SDK is provided, and it's confirmed to support IPv6 - so eventually the ESP32 Arduino will support IPv6. Personally I doubt that ESP8266 on Arduino will ever get IPv6, unless espressif release all the source code for the SDK
- Jan