I wonder about this every time I see a smart TV-related thread on HN. I recently purchased an LG OLED (C5 48") because my old TV died so I'll finally comment. As others have said, just don't connect it to the internet. But you knew this already, so I'll provide my anecdote on the experience of this since I wondered the same thing for years before getting this TV.

When the TV is never connected to internet, and you use a single HDMI source like me, the TV acts completely like a dumb TV. It gets turned on via my AppleTV remote and displays the picture 1-2 seconds later. No LG logo (I disabled this), and no smart interface shown whatsoever.

If you want to change settings, you can display the settings interface via LG remote control and it generally acts like a dumb TV (not blocking the entire screen, so you can adjust picture quality and see the result as expected).

I've had the TV for about two months and never been asked to update it or shown any ad. The only time I've ever seen the smart fullscreen interface is when you unplug a live HDMI source and the TV detects that nothing is there. (If you turn the source off, it tells the TV to turn itself off as well.)

Hope this helps since it's a lot easier to buy a nice smart TV and do it this way than find a truly dumb commercial panel.