One of the new features included with the Pixel 2 and Pixel 2 XL is the Always On Display. The feature has not officially made its way to older Google devices like the Pixel, Pixel XL or Nexus 6P, and it may never since Google likes to give their fans encouragement to buy new devices with exclusive features. That being said you can now Enable the feature on the 6P, Pixel and Pixel XL thanks to Android 8.1 and Substratum even without root. There is a complete tutorial at the link below. If you are interested in this feature it seems to be pretty easy to enable.
via XDA