I bought a new oil boiler 2 years ago that recirculates exhaust gases and vapourises the fuel at ignition point so that it burns much more efficiently with a blue flame rather than the yellow flame used in other boilers. It was around the same price as boilers using the old technology too so it seemed a no brainer.If you're talking domestic gas or oil boilers economisers could be fitted, they are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the cold water used to fill it (the feed water). So far as I can find out it's a question of scale and temperatures. These boilers will become obsolete as new technology takes over sooner rather than later.
It's in widespread use in process industries. I've been involved with waste heat recovery projects a number of times in my career it's a technology that requires investment and research. The capex needs to be justified as a cost plus, spend x get x plus. How that's calculated is complex and often short termism overcomes long term gain and wider aspects not in the equation. Accountants vs engineers, now days the money men win.
I'd say I've reduced my oil use by 10% a year which I'm pretty happy with.
I don't really know why other manufacturers aren't following suit. My boiler is made by a South Korean company