Actually it already exists. My microwave have a button labeled “AI” which suppose to run the microwave until it determines the food is ready.

If it has a humidity sensor it'll likely work, but doesn't need "AI"

Any logic where you could use an if condition will be marketed (probably already is) as "AI".

We have had this function for dozens of years in Japan already

The world has had it, you just have to buy it.

https://www.youtube.com/watch?v=UiS27feX8o0 (it's Technology Connections about a really old microwave that does this well)

Mine have had that for as long as they've existed.

Does it send the zero-time notification to a cloud LLM endpoint that will send back a 10 kB JSON that instructs the oven to turn off? (Or not, how could we predict it?)

Every microwave turns off when the timer reaches zero. It'd be better to have AI that turns it off when the timer reaches one so that I don't have to quickly stop it before the bell goes off. Better yet, a mute function would do the same thing.