Ethernet over HDMI is definitely a real thing. Someone else would have to comment on whether it's enabled by default on various devices, and which devices are capable of acting as a gateway vs client.
BUT ... since we're talking about it ... are the network lines on HDMI reserved for that purpose such that you could create an "HDMI condom" ? I have USB condoms that I use to charge phones with, etc., and wonder if the same concept works with HDMI ...
HEAC utilizes two lines from the connector: the previously unused Reserved pin (called HEAC+) and the Hot Plug Detect pin (called HEAC−).
- https://en.wikipedia.org/wiki/HDMI#HDMI_Ethernet_and_Audio_R...
So if you don't need ARC, it should be possible to make a simple filter that blocks both HEC and ARC. Older HDMI cables that might not connect the Reserved pin might also block HEC+ARC. And it also seems like you can get ARC-only by disconnecting the Reserved pin.
I'd guess HEC is most commonly going to be used in situations where you'd also want ARC -- to provide network access from an A/V receiver to a smart TV, and then get the audio back into the receiver.