You know that's a great question. I'm not sure, but my best guess would be sometime in the late 80's. I tried a quick google search and didn't find anything, though I'm sure someone better could get that answer. I know I'd appreciate the info.
BTW- BEFORE that rule change an offensive lineman was supposed to keep his hand stuck to his chest. Therefore his blocking surface would be measured from elbow to elbow. If you locked out your arms it was holding.
The technique was completely different. The object was to keep you head glued to the rusher's chest. So instead of creating space with your arms, you popped him in the chest with your head and moved your feet to stay in front of them.
I was thinking back to all my football playing days which accounted for 4 years in HS, 4 years in college and 2 after college, and I couldn't think of a play I made on either offense or defense that I DIDN'T lead with my head. When you blocked you fired out low with your head trying to put you facemask into the defender's chest. Then you were supposed to slide your head to the side you wanted to create the hole, and turned you hips into the block using your head and elbow as your blocking surface. Your target point was always the waist.
I kind of makes me wonder about CTE, because if hitting with your head (even in the proper position) causes it, then I'd be the poster child for CTE, because that's all I did.