As a matter of fact, there have been unofficial builds [1], so it's not like the newer Pixel hardware isn't supported at all. Although being Exynos-based likely doesn't help.
Besides Graphene, there's also CalyxOS [2], which is basically like LineageOS with MicroG [3].
And for those who want GApps, they might like their Pixels on stock OS. In fact one of the more popular custom Android flavors these days is Pixel Experience, [4] trying to port these features to other devices.