There are two related, but distinct use cases, which we are in danger of subsuming under one skunked tag. Let’s call them by their originally proposed names:
highway=scramble: a route that requires use of hands and/or specialized equipment. Therefore, it is likely hazardous even for hikers, and totally unsuitable for any other mode of transport. Also, the definition may overlap with the existinghighway=via_ferrata.highway=pathless: a route which is not visible on the ground and is only drawn as a suggested connection between endpoints (see also waterway=link). It may be indeed an unmarked alpine route over scree and rocks, but it may also be a nice stroll over a sandbeach or a meadow where pedestrians can roam wherever they please.
While I think that both of those types should be excluded from definition of path, I’m unsure how best to approach classification, since trail visibility and difficulty are generally orthogonal.