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The Google Pixel 2 and Pixel 2 XL premiered Always-on display on pure Android, but the functionality didn't trickle down to the original Pixel and Pixel XL.
Yet, it's now possible to enable Always-on display without root on the Android 8.1 Oreo developer preview on the Google Pixel, Pixel XL and even Huawei Nexus 6P.
The process is a bit technical and requires some time and studious following of instructions. You can find a full guide in the source link.
The short version is this: you need to setup a theme manager app, buy a $1.99 plugin for it, install the Pixel Enabler app from the Play store and follow some instructions. If that doesn't sound like much proceed to the detailed instructions.
Source.