There's been releases ever since that improve battery life, or, keep it the same while doing more; background apps get throttled, graphics rendering has been optimized using Metal, and in future Mac devices, I'm sure they'll put their mobile processors in it to handle background tasks - it might even be strong enough to power the next generation of Macbook Air.

Mavericks was a big one for extending battery life. It came quite a bit after Snow LeopRd and introduced timer coalescing which significantly improved battery life across the board for all MacBooks:

https://m.imore.com/mavericks-preview-timer-coalescing