They are still alive and well in slum housing. I know several people who have gas heaters in the shared bathroom of their block of bedsits.

I have one and I live in the UK, so the definitely do exist. Of course, I may be unique.

You presumably have central heating in your house, and hot water? If so, you probably have a boiler. And this boiler probably runs on gas!

The comment was referring to "gas fired bath water heaters", which appears to be a specific type of heater fitted in bathrooms for heating the bath water…hence the potential for increased CO risk in the bathroom.

My hot-water + heating system is gas-fired, but the boiler is in a utility closet vented to the outside.

We have central heating but the boiler runs with a heat pump driven by electricity.

You are of course right in pointing out that central heating often goes along with central water heating and that heating is often based on fossil fuels such as gas.

All the people who had them died of CO poisoning.