I doubt anyone has provided reliable data to support that. People make such statements, but the number of manufacturing products sold and used in the US and elsewhere is much higher now than decades ago.
Nobody was producing computers and tablets and cell phones and walkmans and a large number of other products in most every home today that didn't even exist forty or fifty years ago. And things like the number of vehicles and TVs owned by the average family is signicantly higher now than decades ago.
Here is the stat we need:
How many manufacturing jobs were required to produce all the manufactured products sold in the US in 2015, and say, several different years going back to the 1960s?
I don't believe for a second that it takes fewer manufacturing jobs to produce all the manufactured products sold now compared to any prior decade.
I don’t think you need reliable data to support that claim. It’s kind of self-evident. Just go to any large-scale U.S. manufacturing operation and see how few people it takes to work there compared to a generation ago. You used to have entire company towns where thousands of men would work at the local plant. Today, these plants produce just as many units of whatever they produce, with a much smaller work force.