In this tutorial by Blender Guru, Andrew re-unwraps his plane (because he changes the UV to be a 2*1 aspect ratio and so it stretched his UV). He also mentions that with cycles, it won't re-unwrap it. You have to be in blender internal to re-unwrap it.
In Blender 2.8, there is no blender internal anymore. (and with EEVEE and Workbench it doesn't sucessfully re-unwrap it either)
How do you re-unwrap an object and have it be the same scale?