The Nexus devices of old were some of the best phones when it came to custom ROMs, and while the Pixels are still popular options for software mods, CyanogenMod/LineageOS has never offered official builds. I'm not sure why, but I assume it had something to do with the A/B partitioning scheme, or perhaps there was not a maintainer available to work on it.
Regardless of why it wasn't available before, official LineageOS 15.1 (Android 8.1) builds are now live for the 2016 Google Pixel (sailfish) and Pixel XL (marlin). The new Lineage Recovery is being developed primarily for A/B devices like the Pixels, but it doesn't appear to be required - the official flashing instructions still say to use TWRP.
It's great to see LineageOS finally arrive on the original Pixels, especially since Android P will likely be the last major software update for both models. Hopefully the 2017 Pixels are next.