I have mapped thousands of nordic/cross country pistes in OSM. My thoughts:
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piste:grooming=*is a physical attributes which belongs on the way, not the route. Grooming is often not the same for different sections of route, typically if there are multiple routes across a wide network of pistes. -
piste:difficulty=*is widely used for downhill pistes and it seems to reasonably well defined. I never map nordic/cross country pistes with this tag because it is not well defined for cross country. However, resorts sometimes refer to routes as being “easy” or for “advanced” skiers, so perhaps the tag would make sense on the route relation, as a rough guideline for the entire route. I have never seen “difficulty” indications used on the ground for different sections of a cross country piste (in my region, the Nordic countries). -
Routes in my region are often identified by colour, so that could make sense on the route relation.
Example of routes in a resort below:
