Support in cables is nearly universal, because the same pins in the cable are used for the Audio Return Channel feature that allows a TV to pass audio from its internal apps back out to a soundbar or receiver via the same HDMI cable it takes signals in on from other devices. As far as I'm aware no one has made a cable lacking those pins since 1080p was still the high end.

But yes, support for the ethernet mode as far as I'm aware was never actually implemented in any devices that reached retail availability.

How much would support increase when manufacturers realise how few of their TVs don't get connected to the network?