There was another thread with a paywalled article, but here's the actual study that found that smart TVs use "automatic content recognition" to build an ad profile for you based on what's on your screen... including HDMI content streamed from a laptop, game console, etc. Yikes.
At a high level, ACR works by periodically
capturing the content displayed on a TV’s screen and matching
it against a content library to detect the content being viewed on
the TV. It is essentially a Shazam-like technology for audio/video
content on the smart TV [38]. ACR is implemented by all major smart TV manufacturers, including Samsung [9] and LG [55 ].
Our findings indicate that (1) ACR operates even
when it is used as a “dumb” display via HDMI; (2) opt-out mecha-
nisms stop ACR traffic; (3) ACR works differently in the UK as com-
pared to the US.
So it seems like you're opted-in by default, but you can stop ACR traffic by simply configuring six different options on Samsung, or eleven different options on LG.
Oh, and this doesn't seem to happen when you're using native streaming apps like Netflix or Disney+, because hey, they wouldn't want to infringe on those companies' rights by spying on them, right?
My TV isn’t even connected to wifi!
It's worth keeping in mind that your network is not the only network in the world, and WiFi is not the only kind of link.
Neighbors exist. Open guest networks exist. Drive-by and fly-by networks exist. Mesh networks exist (and are already created by devices like Amazon Echo for use by other devices in the neighborhood). Special-purpose networks quietly created by internet providers using their CPE exist. Satellite links exist, and have been getting smaller and cheaper; maybe enough for a TV before long. Power line networking exists, and can reach across property lines. Bluetooth, LoRa, cellular, etc. etc. etc.
I'm not suggesting that all smart TVs make use of these things today, but some of them are already capable, and since the capabilities are increasingly cheap and easy to implement, they will almost certainly become more common in the years to come. Let's also remember that behavior that is not documented or enabled at purchase time can be enabled later, and sometimes is.
If I were buying today, it wouldn't be a smart TV. I would instead look at gaming console monitors, computer monitors, projectors, dumb TVs, and commercial displays. That way I wouldn't be showing manufacturers that spyware appliances are okay with me, nor giving them money in support of spyware product lines.
If I already had a smart TV and wasn't in a position to replace it with something trustworthy, I would mod it: Open it up, find any network-capable components inside, and physically disconnect them. (Or if I didn't have those skills, I would get a qualified friend or electronics repair shop to do it for me.)
Good luck finding one of those.
They'll make a comeback in a few years when people with Google's advertisement TVs start having to buy whole new TVs because the cheap Android tablet built into the TV will age like a cheap Android tablet and start struggling with basic tasks like browsing content.
No they wont.
There is no demand for dumb-TVs. People are stupid, people don't know what they want, people don't know that their TV is broken because the manufacture made it broken. People will buy what they are told to buy.
At best you can buy a smart TV, hobble it, and use your own video sources.