In Android Oreo, Google introduced a new way to "shame" apps, for lack of a better term, that were running in the background unnecessarily and draining your phone's battery. The goal was to give a persistent notification that said "[app] is using battery" whenever that app was running in the background without actually providing anything to you, in hopes that you would track down the issue and stop it ... at least, that was the idea.
In reality, the "[app] is using battery" notification was often persistent even for apps you wanted to be running, like a music or podcast app — and in many cases you'd continue to receive the notification even when you thought you had stopped the app or adjusted whatever setting you thought was causing it. It was a bad user experience, and didn't end up having the desired effect of making app developers more conscious of their apps' battery usage. And so, Google scrapped it entirely. The persistent app battery drain notification is gone in Android P.
This 'feature' needed to die — and Android P brings better controls and information on battery.
Google didn't just change how the notification mechanism worked, update the wording or have it launch a different settings pane — it killed the "feature" entirely. The "What's new in Android" session at I/O 2018 pointed out the change, receiving a round of applause from the developer-heavy audience. Nobody wanted this, and I got the feeling that the Android team at Google didn't even want it anymore.
But that doesn't mean Google isn't focusing on battery life in Android P — in fact, it's doing more than ever to improve longevity. "Battery saver" is still here and can be turned on automatically at any percentage between 75 and 5%. A new "Adaptive Battery" feature learns how you use your phone and extends battery life by limiting battery usage from less-used apps. There's further information in the battery settings page showing how long your phone should last on average and what apps are causing any problems.