Oreo/Pixel2: Bottom Navigation Bar is no longer fading out
Credit to PM_ME_2LEARNHOW2CODE from r/GooglePixel:

To autohide statusbar only, use command:

adb shell settings put global policy_control immersive.status=*


To autohide navbar only, use command: 

adb shell settings put global policy_control immersive.navigation=*


To autohide both nav and status bar, use command:

adb shell settings put global policy_control immersive.full=*


To revert back to stock, just re-enter the command you did but without the *

adb shell settings put global policy_control immersive.full=


If your phone doesn't let you unlock it/freezes up when you try to unlock and you want full immersive mode run these:

adb shell settings put global policy_control immersive.full=*
adb shell settings put global policy_control immersive.full=apps,-com.google.android.googlequicksearchbox


I personally used a tweak to permanently hide my navbar and then use immersive for my status bar.