While its allowed by the standard, its not something that is often used. Certainly the AppleTV that I use does not even offer an option to share its network connection over Ethernet. And I’m not aware of any other box that does.

Pretty sure you need a special cable that carries ethernet, as well as a device at the other end that supplies ethernet to HDMI, so you have to really want to let your "smart" tv to have access to the internet.