I assume (I’m a more recent participant in tagging discussions) that the main reason to split off trail_visibility from the hiking difficulty grading system we use here is that in the original SAC scale, they were assumed to change in parallel, i.e. a more difficult trail is also assumed to be a less visible trail. However mappers found many examples of trails that were easy but not very visible (a trail across a meadow, for instance, is often easy but difficult to see) or difficult but easy to see (a via ferrata for instance).
The sac_scale key we now use for hiking difficulty still is a combination of technical difficulty (how much you have to lift your feet, whether you need your hands, etc.) and danger (exposure, risk of falling). @erutan proposed a few months ago to split these as well (here) but this was not received well.
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