Users of the XDA Developers forum under nicknames and LuK1337 luca020400 ported to the OnePlus 6 16 version LineageOS based on the Android Pie.
The informal Assembly carries with it all the innovations of «fresh» version of the OS, as an update of the navigation system and interface design, technology, Adaptive Battery and so on.
Before installing the firmware, it is worth considering what it is, some device features may not work. For example, LineageOS 16 6 OnePlus does not record video. Also there are some «chips» LineageOS like Live Display and IMS.
Installation instructions available here (in English).