I fail to understand your interest in the tracktype key at all. Didn’t you mention some time, you stopped using it? Could you elaborate on that?
In my local area, by the end of 2014 most tracks were mapped and lots of them given a tracktype. This is a very small area, but the length of tracks it contains spans half the globe 
The progression that the images show then and now is fairly easy to grasp: How much effort went into the construction to make this track usable for a limited number of people in their errands. The term “firmness” was not even mentioned then. And the other topic to give a clear definition of “firmness” also not going anywhere.