Considering the number of slaves you'd need to operate your own hypocaust system, I would classify that securely in the luxury category for antiquity.

Wealthy Romans in palaces yes. But when I wrote "our" I meant it in the more universal sense.

Conversely, some impoverished people in the US today have very inadequate heating systems in their houses, and resort to things like propane heaters, or running their stoves and ovens to heat the house, which can cause carbon monoxide poisoning indoors.

That’s based on low population densities, and inequality in terms of commanding labour (getting slaves to cut down trees and keep the heating system for a large house running) and in accessing natural resources (a house having enough acreage of land to grow enough wood to keep the heating system going).

It’s something which is a luxury in the absence of fossil fuels, or the rollout of new technologies they mention.