Since the late 1970s early 80s. 1981 maybe before. Us old folks will remember the
V-8/6/4 Caddy 
see if I can find you a link to some info
"
4. Cadillac V-8-6-4 - In the early 1980s, Cadillac started to feel the heat from Mercedes-Benz and BMW. Already bruised by the diesel debacle, Cadillac wanted to make a grand engineering statement.
The V-8-6-4 - GM's first attempt at cylinder deactivation - did make a statement, but not the kind that Cadillac wanted. The engine bucked and jerked. The transition of the cylinders turning off and on was so rough and unreliable that it angered many Cadillac owners.
But the idea behind the V-8-6-4 - to shut off the flow of oil to the lifters and close down cylinders - was a good one. Unfortunately, the sensors and computing power to make the system work seamlessly just wasn't available back then.
Flash-forward to today. GM is in production with its Displacement on Demand cylinder-deactivation system on its mid-sized SUVs. Powerful computers shut off the flow of oil to the valve lifters, which turns off half the cylinders when at highway cruising speed. This gives about a 5 percent fuel economy gain and costs GM just $50 per engine to install.
The system works so smoothly, drivers don't know it's there. Chrysler has a similar system for its hot-selling Hemi V-8s."
http://www.autosafety.org/article.php?scid=77&did=1091
"A terrible idea in 1981 may be the solution for increased fuel economy in the new century.
We're speaking of the short-lived Cadillac 8-6-4 engine that proved a major flop in 1981. General Motors, which suffered a major black eye with the 8-6-4 failures, is looking to bring back a version of that engine.
GM's Powertrain Group's executive director for advanced engineering Fritz Indra said he believes the time is ripe for cylinder deactivation, the technology behind the old 8-6-4. Mr. Indra said it is the simplest and least expensive way to improve fuel economy. "
http://www.highbeam.com/library/doc0...eyword=&teaser=