I don’t know about progress bar, but actually a kettle with a visible thermometer would make a lot of sense -and there’s no reason why it wouldn’t be immediately possible.
Water boils at 100 degrees, so it’s much the same as a progress bar anyway. I suppose a kind of progress bar could be made using the current temperature, but that wouldn’t necessarily indicate how long more it would be boiling for since I imagine it would boil quicker towards the end. Still, I suppose it would be a progress bar. 🙂