Yeah, so? I’d actually want an above ankle-deep mud and vaguely threatening cows to be mapped with highway=demanding_path (or whatever value it happens to be).
Actually, for my use case I’d prefer highway=grandma_hostile_path, but I don’t really care about name.
(It could be highway=demanding_path or highway=scramble or highway=Q123456, as long as it would never lead poor grandma there - nor even show her that this way exists!)
What would be the primary purpose, as others have noticed, is for that tag to stop being rendering and routed everywhere by default. Only after intentional work by mounteering-app-developer (or whoever) should it become usable (and then hopefully rendered differently - but even if not, it would still fulfiled its primary purpose).
The purpose being, when grandma does open www.osm.org or their granda-routing-app, she will only see nice walk in the park paths, and never be presented with other type of path: “yeah, any ironman finalist has almost 90% chance to pass this, but it is still highway=path. See, it has also ironman_scale=90% and smoothness=impassable and sac_scale=T99. Neener-neener-neener!”
I absolutely agree with that part! Exactly what I am talking about!
Although, there are no mountains here, so you might be wondering why a I care a lot about highway=scramble or similar. I’d be marking ways with halfway-up-to-your-knee-mud and be-prepared-to-be-mightly-scratched-to-get-through-those-thorny-bushes. Hey, you have to use your hands or you’d lose an eye, so it is kind-of-scramble, right?
Yes, I currently map them with: highway=path + surface=mud + informal=yes + foot=discouraged + smoothess=impassable + trail_visibility=horrible + sac_scale=demanding_mountain_hiking + obstacle=vegetation + note=do not go here, and you know what?
www.osm.org and mostly everything else still navigates my grandma there.
That is my use case. Now, before I get beaten to death (appropriately using both foot and hands 
) with “yeah but we don’t care about that but only about scramble!”, do note that it does not preclude that! Instead, highway=grandma_hostile_path (or highway=demanding_path or similar) only widens the possible usages.
Extra tag demanding_path=scramble would when used in combination with highway=demanding_path would mark scramble exactly as well as highway=scramble!
But it would also leave open the possibility of having demanding_path=jungle instead, or demanding_path=knee_dep_mud, or demanding_path=corrida, or demanding_path=active_volcano. (thus widening the circle of interested users even if they don’t particularly care about mountains and stuff)