Around 07-08ish. For all I know it could have changed again. I have not been there in over a decade.
The huge change (with the singing cows) was in 2016. There were some minor changes prior, but 2016 is when most of the real education side was lost. If I remember right, the 2008 change was just the beginning section about harvesting the cacao, but I may not recall correctly. We had season passes from 2006 to 2019 as our daughter grew up and loved Hershey. But we were very upset at the big change in 2016.
I just went on it this past Spring and it has continued to change a little since that big change, but never for the better. And upkeep is absolutely terrible.