Today an other smartphone beside the OnePlus 2, 3/3T and the Xiaomi Redmi Note 4 has received an unofficial Android Oreo update:
The Moto G4 Plus (aka "athene")! It received the unofficial Android Oreo update through the custom LineageOS.
The developer rahulsnair has posted the early state ROM on XDA.
The most important functions are working (like RIL, Bluetooth, WiFi etc.), but there are many still unsolved bugs, so an use as daily driver isn't recommended.
That proves that even before the full publication of lineage-15.0 source code it's possible to create more or less working LineageOS builds.
I hope that really soon the full LineageOS source code will be published so that everyone can start to build and develop.