Is it just disabled or gone? If only disabled you should be able to go into Settings> and find either Battery or Power Saving to try and enable the function.
As phones become more hi-tech they begin to suck up more battery power to provide faster speeds/more functions so while comparing your M8 to the M9 seems reasonable, they are two different phones.
Fair enough, but it still doesn't explain why the Verizon 'variant' of the M9 would purposefully drop a standard Android feature like Power Save Mode.
Honestly, I'm just curious (I like the phone regardless so even without the feature I'm cool with it). So to answer your question, the 'accomplishment' would be to have the question answered