Border Nether Lines Have Been Added
I just finished up the border nether line route, it runs along each respective 4000 axis, equating to 32,000 in the overworld. You can access the route by way of any of the main lines, just head to the nearest +/- 4000 and you'll hit it. The line colour is white + whatever other main line it runs parallel with, this is so its easier to figure out where you are on the line. For example, if you're on the western border line running north/south, the western wall of the tunnel is white and the eastern wall is orange, matching the parallel main line. The nether map has been updated with them (though they're not quite placed correctly, they're within like 300 blocks cause the map isn't big enough).