John J presented the following explanation :
If installed in a correctly ventilated room as far as I can recall the old
thinking was the rate of gas flow through the burner jet was insufficient to
raise the concentration of gas in air to attain explosive levels and the
stench would alert person's to the gas escaping.

Such ventilation is often deliberately blocked up these days, relying
upon flame failure devices for protection and room sealed boilers. No
problem at all, if there are no products of combustion or gas escapes.