Trail_visibility was introduced as a companion to sac_scale key more then ten years ago. As today got mentioned here, Use of sac_scale in non-mountainous areas by @rhhs the link is no longer prominently showing in the documentation.
As a matter of fact, the Mountain Hiking Scale developed by the SAC/CAS does not have that concept of trail_visibilty - this is a fully original OSM notion. Let me explain: The SAC scale talks about routes over paths and pathless terrain. The higher the grade, the less path, the more pathless terrain will be travelled along the graded route - exemplary ASCII art:
=============== T1 Hiking
--------------- T2 Mountain Hiking
---- ---- ---- T3 Demanding Mountain Hiking
--- --- ---- T4 Alpine Hiking
--- - - - -- T5 Demanding Alpine Hiking
--- - - T6 Difficult Alpine Hiking
= means a trail well cleared (Weg gut gebahnt T1)
- means a continuous trail (Durchgehendes Trassee T2)
A blank means pathless terrain (intermittent Trassee?)
The higher the grade, the less path there is. But not in the sense of a gradient in visibility, but in the binary percentage of where there is a path, or where there is no path along the route.
That is why the documentation no longer mentions SAC scale.
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Ce sujet de discussion accompagne la publication sur https://community.openstreetmap.org/t/trail-visibility-explained/104726