LineageOS is the most-used custom Android ROM around, but that doesn't mean there isn't room for others. Paranoid Android was a popular choice back in the KitKat days, and while the ROM returned in full force with Nougat builds in 2017, there hasn't been much news since. Now the project is back (again), touting Android 10 builds for nine phones so far.
LineageOS is the most popular custom ROM in existence, and the project prides itself on bringing newer versions of Android to unsupported devices. However, Lineage has been a bit slow to roll out a version based on Android 10 — the Pie-based ROM was already available by this time last year. Thankfully, the next major version of LineageOS seems to be just around the corner.
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.