It seems that the Lineage OS team will not release LineageOS 15 we are all expected. The team is now setting up to release LineageOS 15.1 based on Android 8.1 Oreo, as Google has released stable builds of Android 8.1. In a recent post about changes to the latest official builds, the Lineage OS team informed that they are moving straight to the LineageOS 15.1 builds by skipping the 15 builds.