My FIRST DRAFT for goals for highway=* elements would be:
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All kinds of routes existing in any way shall be supported.
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Like with the Wikipedia rule, they must exist or be defined in some way outside of OSM, typically visible on the ground (at least trail traces, trail signs, cairns, pitons, poles on a glacier) or regularly following the exact same route (for instance a snowmobile route used to supply an alpine hut in winter). I am not used to snorkelling or diving, maybe there are standard routes defined by any means and not just “hello world: this is where I crossed the ocean in 2024”
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Routes can be given various attributes, for instance for the type of surface or the beauty of the scenery or whatever. But they must receive minimum tagging for security and accessibility, if they are not a safe walking path, or a road suitable for saloon car, or access is not permitted to the general public (on private land in countries like the USA, South Africa etc.). Of course, polygons marking non-accessible private land, national parks etc. are an alternative and equally sufficient solution to tagging individual paths.
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The minimum tagging for security and accessibility should be globally standardized and simple to allow apps or map renderers to automatically filter and/or mark the routes worldwide. This means that a non-standardized tag like “for experienced alpinists only” is a no-go and a multitude of difficulty grading systems like the SAC scale T1…T6 which originates from Switzerland, but then Switzerland’s official trail difficulty blazing yellow, red, blue, UK grades 1, 2, 3, etc. (Trail difficulty rating system - Wikipedia) are a high burden for apps, even without considering mountaineering grades like the French Alpine System F/PD/AD/D/TD/ED, the UIAA I … XI, or the many more climbing grade systems. A global (OSM) standard may simplify things to some extent, because the different grading systems cannot simply be mapped to one another as they take different aspects into the consideration or balance them differently: technical difficulty, mental challenge, length/duration of the difficulties and remoteness/isolation (discovery and rescue chances in case of failure).
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Each route being mapped shall create some advantage to some public user. I am not sure about all the many routes in a busy climbing garden starting all two to three meters from one another, sometimes leading diagonally or even crossing one another. I would be afraid of an unreadable mesh of lines. In a single-family home neighborhood, the many small paths and stepping stones along the houses from the front garden to the back garden are also not worth mapping. Sometimes less is more.