LineageOS is arguably the most popular custom ROM on the planet, with more than 1.6 million active installations. However, there have never been official builds available for the OnePlus 6, possibly due to the phone's use of A/B partitions. That has finally changed, as the first nightly downloads are available for the OP6.
It seems the Lineage team is catching up on its backlog of devices, as support for the Essential Phone was just added a few days ago. There were plenty of unofficial builds available previously, but now the ROM is hosted on the project's servers and should be relatively bug-free.
You can find installation instructions and downloads at the source link below. It's important to note that this doesn't include the OnePlus 6T.