Android Oreo 8.1 based LineageOS 15.1 official builds have started from February 2018.
This is quite obvious because the Official LineageOS 15.1 wants the maintainer of a build to be compliant with the LineageOS device support requirements. Which means, on an Official LineageOS 15.1 build you have all the major hardware components are ready to use out of the box.
On the other hand, an unofficial LineageOS 15.1 build is from an individual developer (or sometimes a group of developers) without any standard in place. I mean, it’s possible that in an unofficial LineageOS 15.1 you have major hardware features (RIL, Camera, Wifi, Sound etc.) are not working and is released to public to use (but of course always with a WARNING message).
LineageOS 15.1 for Google Pixel and Pixel XL:
The official LineageOS 15.1 NIGHTLY builds for Google Pixel and the Google Pixel are scheduled on the LineageOS build roaster for Tuesday of every week. Which means, on Tuesday of every week Google Pixel and Pixel XL users will get a weekly NIGHTLY build update of LineageOS 15.1.
How to Update Google Pixel and Pixel XL to Official LineageOS 15.1:
I have captured a step-by-step guide on how to update both the devices to LineageOS 15.1.
Follow the below links to update to install LineageOS 15.1 on Google Pixel and the Pixel XL:
What if you found a BUG on the LineageOS 15.1 build on your Google Pixel or the Pixel XL:
It’s possible that you may face some problems on these LineageOS 15.1 NIGHTLY builds as these are not yet stable.
IN case you get a BUG on the LineageOS 15.1 build on your Google Pixel or Pixel XL then you can always open a jira ticket to the LineageOS team with all the log, so that they can address the issue.