Though, it's true that occasionally tracks degrade so much they only look like narrow footpaths, in which case I suppose a downgrade would fit fine. 
Suggestion: 
1. If vegetation (eg. bush/tree branches) is enroaching upon the way from the side, but does not grow on the way itself: obstacle_vegetation
2. If vegetation grows from the way itself (eg. bushy grass or bushes or even trees), truly making the fabric of the way itself narrower: change highway type