There was a guy posting a while back about that. His claim was that O2 sensors are an item that should be periodically replaced, even with no codes. Sort of like replacing plugs. I think I saw another post about how you can get the data the O2 sensors are providing and make a decision about how "healthy" they are. The guy claimed it can be a big hit to MPG. Pretty expensive to replace I think so it seemed like money chasing money. I've been curious about that since I saw it.
I would say that mostly that was just a claim.
People have been lying about fuel mileage for decades.
The auto parts stores and manufactures like Bosch have been saying you should replace your oxygen sensors every so many thousand miles for fuel mileage also but they have a vested interest in selling oxygen sensors.
I'm sure they can get lazy but I would hook up a live monitoring scan tool and watch what they're doing.
If I were going to swap some out to test I would make sure I bought some very economical ones on eBay.