HDMI is heavily used for ACR (automatic content recognition) in smart TVs:

"Our findings indicate that (1) ACR operates even when it is used as a “dumb” display via HDMI"

"For both LG (a) and Samsung (b)TVs, the scenarios with the highest ACR traffic are Linear and HDMI."

* https://dl.acm.org/doi/epdf/10.1145/3646547.3689013