I have an AppleTV 4K connected to a Sony screen over HDMI. The AppleTV is connected to the Internet via WiFi, the Sony is deliberately not connected - just power and HDMI. Recently I have learned that there is such a thing as ethernet over HDMI. How can I confirm that the AppleTV is not providing this to the Sony? I don't want the latter to phone home, "upgrade" its firmware, run Android apps or anything else - I just intend for it to be a dumb (i.e. non-smart) screen.
HDMI Ethernet Channel never took off, and no commercially available devices support it. Your Apple TV, HDMI cable and TV would all have to support it, and it’s almost certain that none of them do.
See AVforums' List of HEC and ARC enabled devices? discussion.