I was able to do this with my TCL/Roku TV a few months back. My curiosity got the best of me with the announcement of their latest OS, so I upgraded to see what the new features were like. Now I am stuck in the ecosystem, as the downgrade feature is completely disabled (even from a USB stick, with no internet connection, after a factory reset). Luckily, I use a Pihole as my DNS on my router, but still. Super annoying.
That's a valid point, dumb TVs need to be kept dumb by not updating/upgrading the OS although this results in conundrum of not patching existing vulnerabilities in favour of not introducing new vulnerabilities? Vulnerabilities of dumb TVs could still be exploited by plugging in USB devices/SD card or even STB like Roku.