The Pixel 4 has a number of annoyances, but depending on how you feel about constantly visible battery status icons, you may or may not like this one. When you're watching a fullscreen YouTube video on the Pixel 4, you might see a dim battery indicator in the top right corner. Getting rid of it is quite counterintuitive: you need to deactivate Motion Sense's "skip song" feature. Conversely, if you'd like seeing the battery indicator while watching YouTube videos, you need to turn on the Motion Sense feature (if you haven't already).
Turn off the "skip songs" feature in the motion sense settings.
— Jibin Johns (@Jibinjktda) November 7, 2019
We can only speculate on why these features are related. It might have something to do with heads-up notifications. When they pop up on top of your immersive videos they're also accompanied by a little battery icon, just like the one that's always visible on the Pixel 4. Motion Sense displays a little animation on top of the status bar when it's available, and since heads-up notifications appear in a similar fashion, the underlying technology might be the same. YouTube is one of the only video apps to support Motion Sense, which is why you don't see the persistent battery icon when watching fullscreen content in other apps.
When you receive a notification during video playback, a similar dim status bar will show up.
In previous Android versions, the related status and system bar dimming API had the tendency to go haywire. I remember that the dimmed battery icon would sometimes stick around long after pop-up notifications disappeared, leading to a similar always-on situation. Since that doesn’t seem to happen on Android 10 anymore, we can assume that Google might fix the current issue later on, as well.