I think @Nadjita here pointed out an important aspect in which (some) access-tags would differ from suitability. Namely the fact that ‘suitable’ or ‘practical’ or ‘useful’ in a sentence like ‘x is suitable/practical/useful’ do not yet fill a predicate. Ways are not ‘suitable’ (or ‘unsuitable’) as such or in the abstract, but always suitable or unsuitable (etc.) for particular purposes or ends.
With that in mind, we already have highway=track and tracktype=*, I suppose mostly for four-wheeled vehicles. trail_visibility=* and sac_scale=* are mainly for pedestrians and MTBs. So what would be the main target audience for a suitability-tag? Non-MTB bicyclists?
Presumably pedestrian routers would already steer clear from paths or even footways with no or bad trail_visibility and a high sac_scale? Ditto for non-4WD motor vehicles and tracks. I suppose it would be nice to have an indication that a path is narrow enough that normal handelbars will bash against trees or shrubs, but I can already glean that from a width estimate. I won’t take a narrow-tyred bicycle to a path that has a surface of =gravel (or no surface tag at all). Perhaps help avoid bicycling through very congested thoroughfares? I already avoid bicycling through highways that have multiple bus routes (public transit relations in the OSM world) going through them, and then there’s already the embedded_rails=* key as well.