Broadly I’m in favour of the idea, although probably in the highway=mountaineering form. However, there are other similar “routes” which aren’t continuous marked paths, but may be extremely well known by people interested in them:
- Traditional alpine routes. Hoernligrat, Matterhorn Nordwand, Brenva Arete, Eiger Nordwand. These would typically be graded by traditional alpine scales F/PD/AD/D/TD/ED (French) /WS/ZS/S/… (German) or their equivalents in other countries. Some at the lower end may also be graded with SAC scale, or parts of the approach could use this tag.
- Expedition style ascents in the Himalaya, Andes etc. I’m not aware of any traditional grading for these, and their nature is completely different if one is making the first ascent in season, leading, guiding or being guided. The classic example is Everest via the Western Cwm and South-West Ridge.
- Alpine style ascents in distant ranges. Again I’m not aware of gradings, but keen amateur mountaineers may attempt peaks of 6000m & over (I know at least one OSMer who has done this).
- Mountaineering ski tours: with ascent & descent. Obvious examples of well-known routes are the Haute Route & the Patrouille des Glaciers.
- Downhill ski routes which lie beyond typical off-piste descents. The Mer de Glace is a well-known example with few difficulties, but many objective hazards. It is usually skied with a guide. In the Chamonix valley there are hundreds of these routes, some of which are not only technically very demanding, but involve severe objective dangers. A number of published guides are available for various favoured locations in the Alps, but in other locations knowledge will be limited to the local enthusiasts.
- Bushwacking hiking routes in back country. This applies in particular to parts of North America, Australia and New Zealand, but there are also plenty of places even in Europe (e.g. NW Scotland). I presume in tropical countries where vegetation grows fast there are well known routes which require use of a machete to traverse.
Ideally I’d like a solution which also encompassed the ability to tag these other options. highway=mountaineering would cover most of them (