The highlighting on searched kw's in Y cache has been there as long as I can remember.
The last change I'm aware of is when they also started highlighting word forms, I think sometime back in mid-2005 (so if you searched on only a plural form, e.g., "blue widgets," they highlighted "blue," "widget" and "widgets").
But to be honest it's been a while since I paid great attention to the highlighted words in the Y cache.
Did a little checking just to be sure I wasn't confusing G with Y. The archived content below seems pretty conclusive, unless Y's Help page has consistently been inconsistent with what was actually happening...