Formerly known as CyanogenMod, LineageOS is the biggest Custom ROM development team. They support various devices, ranging from Samsung to Moto, including Xiaomi devices. But, there was no support for the first generation Pixel devices until today.
The two devices have the code name as Pixel aka sailfish and Pixel XL aka marlin. Today devs at Lineage has released the nightly builds for both the devices. There are several builds for first-generation Pixel devices but none of them were official, until Lineage launched them officially, today. Since these ROMs are for first-generation Pixel Lineup, we have no idea if they will do the same for the second generation.
It is the first time Google devices are supported by LineageOS. Since these builds are nightly builds, that means they aren’t stable as of now. You should flash Nightly builds at your own risk since they likely to have several bugs than the stable ones. Nightly builds are experimental builds. just a slight error in the ROM, the ROM support is removed.
But one thing here makes me think – LineageOS is based on AOSP Android whereas the LineageOS is also the same, so why need a custom ROM for such device?
Anyways, if you want to check out the ROM review head to the link here.