Great photos - the second one shows exactly the area that used to be a footpath. You're right about the soil colour. It's clearly darker where the path was. The ploughing must have been deliberate, as the byway was unmistakable and clearly demarcated from the adjacent field (albeit a little overgrown with nettles).
It's also been there for at least as long as I've been visiting the area, which is twelve years.