The Trifecta is complete. Combined with RFC: hiking_technique key (or a better name!) to describe movement on paths by hikers and RFC: hiking_exposure key (along with hiking_technique this disambiguates sac_scale) - #16 by Kovoschiz this aims to have a global scale that can adequately and (relatively) unambigously describe key aspects of hiking paths for casual to experienced hikers. More specialized use can include specialized rating systems.
This comes about after a lot of discussion, healthy debate, and sharing of resources - thanks to everyone that has contributed to related discussions. The snark in here won’t make into a formal proposal, but I got a little tired of writing it. 
Photo examples can be found at the related NFS Class ratings in this pdf.
This is essentially the Orientation part of the German (Bergsteiger Magazine) rating system, where Power is hiking_technique and Psyche is hiking_exposure / hiking_fall_risk. Endurance is covered by modern mapping technology that can spit out elevation gain and length data.
Key Values
1 Excellent
hiking_visiblity=excellent
Surface Characteristics: Often of imported materials like gravel or asphalt, and tends to be wide enough for people to easily pass or walk next to each other which aids in visibility. These tend to be highly trafficked and easily accessible so tread from human traffic (surface applicable) is generally visible.
Construction: Tends to be heavily constructed and signed, with railings to protect from falls. These aspects aid in visibility. Obstacles of any significance tend to be cleared so the smoothness of it if on rough terrain also helps identify the path.
Routefinding/Orienteering: None, this path is incredibly obvious and visually stronger than any game trails, drainages, etc it will come across and will have no true gaps of visibility. The path is distinct enough to still be visible in fairly extreme weather.
Expected Experience: Tourists, elderly, children, people with injuries or disablities, etc can walk with ease and enjoy the scenery.
Mappings: this has no equivalent in trail_visibility rofl, NFS Trail Classes 4 & 5 in the US, CAI T in Italy, AWTGS 1-2 in Australia, and PWS W1-W2 in Tasmania.
2 Good
hiking_visiblity=good
Surface Characteristics: The surface of the path is distinct from the surrounding terrain, but tends to be of natural material and usually of single track width, though allowances for passing may exist and it can be wider. Imported materials may be used when necessary. The path should be more visible than game trails or drainages that it crosses. Large obstacles will tend to be cleared, but there is some significant variance and relying on looking for cleared terrain isn’t a solution.
This is the highest visibility rating that paths with no visible surface characteristics (e.g. the path consists solely of markers on solid rock) can get. trailblazed=* should be used to indicate the type of blaze, and trailblazed:visibility should be excellent.
Construction: Generally moderately constructed to withstand high to moderate traffic, all consequential water crossings are generally bridged and markers are used to maintain visibility of the path when necessary.
Routefinding/Orienteering: While not as obvious as an excellent path, inexperienced hikers have no significant chance of getting lost. Having a map would be recommended. A lack of inattention may cause someone to end up on a short informal spur trail to a viewpoint or water source, but that isn’t a safety concern. Small gaps in visibility (minor creek crossings) have the either the path or markers visible on the other side.
Intended Experience: This is what most people think of when they think of a trail - it’s charming, not in your face, but easy to follow.
Mappings: trail_visibility=excellent aka SAC Yellow, NFS Class 2-3, CAI E, AWTGS 3-4?, PWS W1-T3.
3 Intermediate
hiking_visiblity=intermediate
Surface Characteristics: These tend to be purely single track in width, almost entirely made of non-imported materials, and not as well cleared as good paths. They may become faint or indistinct in places, or itermmitently disappear for short periods of time. The surface of the path when will probably be more visible than game trails, drainages etc that cross it but it wouldn’t be uncommon for it not to be so.
Paths with no visible surface characteristics (markers on solid rock) should be tagged with trailblazed=* to indicate the type of blaze, and trailblazed:visibility should be good or intermediate.
Construction: These tend to be minimally developed and construction generally will not be useful to aid in locating the path. Junctions tend to signed, and consequential water crossings tend to be bridged although they may be of simpler construction than on hiking_visiblity=good paths. Generally the bare minimum of construction is used to help keep a wilderness aesthetic and reduce impact.
Routefinding/Orienteering: Some routefinding may be necessary, but it should be fairly straightforward. Inexperienced hikers may get lost, experienced hikers may experience brief periods of confusion but be able to follow the path. Occasional stopping to “search” for the path may be necessary, but it should be found again relatively quickly.
Intended Experience: This would probably feel like an adventurous path for a hiker with low to moderate experience, and a good way to build up routefinding experience. Experienced hikers shouldn’t have any major difficulties with it.
Mappings: trail_visibility=good and possibly trail_visibility=intermediate (bad is sometimes invisible, intermediate is mostly visible) aka SAC White-Red-White, NFS Class 1-2, CAI EE?, AWTGS 4-5?, PWS T4?.
OSM Note: One of the example photos for trail_visibility=good states that for “50 meters the trail is not really defined. Orientation is very easy however.” The other one shows a somewhat faint path through grass. That’s… not good in any realistic sense of the word.trail_visibility=good is not good. fight me.
4 Bad
hiking_visiblity=bad
Surface Characteristics: Extremely variable. Portions of the route may resemble a fair or even good path, while others will be pathless and on naturally occuring terrain. The surface of the path when it exists may be the same visual weight or weaker than game trails, drainages etc that cross it.
Paths with no visible surface characteristics (markers on solid rock) should be tagged with trailblazed=* to indicate the type of blaze, and trailblazed:visibility would be bad or horrible.
Construction: Little to none, as these tend to be undeveloped, abandoned, or informal path. Minor to moderate water crossings will generally be unbridged.
Routefinding/Orienteering: While there is a occasionally a path, you need to be able to find it again when it’s not there. If you are comfortable with offtrail travel, it’s a bonus when the path is there. If you’re not then this is probably not the path for you.
Intended Experience: This is a “wilder” wilderness experience for people that enjoy routefinding and off path travel, in that mindset it can help having a path come and go to ease travel and navigation through off-path terrain.
Mappings: trail_visibility=no (ambiguous on wiki this would be bad, in iD editor this would be no), trail_visibility=bad, horrible & possibly trail_visibility=intermediate (bad is sometimes invisible, intermediate is mostly visible) SAC T4-5, NFS 1 (in bad shape), AWTGS 5, PWS T4-T5.
5 No
hiking_visiblity=no
Surface Characteristics: None in any meaningful sense there may or may not be occasional signs of tread, but nothing resembling a path. The gorgeous descripton of trailblazed:visibility=no states: “trail markers almost non-existent and/or they are so rare that they are close to useless”. chef’s kiss.
Construction: Nope.
Routefinding/Orienteering: Yes please.
Requirements & Intended Experience: the ability to routefind / orienteer off-path and possibly move over rough or semi-technical terrain.
Usage: This should only be used for commonly used gaps in between paths that have a visibility of bad or higher to allow for routing in applications, or in areas where mapping commonly known pathless routes as paths is normalized. This value is not verifiable and has no ground truth.
OSM Note: there is some interest in having “pathless paths” be an object of a relation of a series of nodes, vs a highway=path. None seems more appropriate than No.
Mappings: trail_visibility=no (ambiguous on wiki this would be bad, in iD editor this would be no). SAC T6, AWTGS 6?, PWS R?.
Thoughts
If anyone has been in the trail_visibility thread the takeaway is that it’s a hot mess. If you haven’t and value your sanity, please don’t visit it.
While I think it’s useful to have some sort of standardized hiking key, I see no problem with people also tagging their local systems. They will be easily understood by locals and people familiar with the region and there is unlikely to be a 1:1 mapping with any generalized or universal key. CAI, SAC, and others can live alongside this in meta-data, and could even be preferred on a regional basis if mappers choose to do so and this used as a fallback.