2020.01.20
Me? I think it's when Lego started putting faces on the back of the heads of minifigs, as well as the front. So sometime around 2002. Yeah, you have more options for expression, and I guess usually the back of the head is behind some kind of helmet or hat or hair blob, but still... for me it's not worth breaking the kayfabe of the "thingness" of the character - that piece is no longer a "head" it's just a brick that you can rotate for the most aesthetically pleasing appearance.
Here's what brought it to mind, I finally got around to putting this Tron Legacy lightcycle kit Diane got me last year...