I think legal access should be a separate thing from practical access. Even when something is illegal, it might be widely practiced at different places, or one might be in an emergency when the legal restrictions might not (even accordign to the law) apply etc.
However, upon further reflection, https://wiki.openstreetmap.org/wiki/Key:mtb:scale covers for bicycles what sac_scale covers for hikers, so I think to both the same thing applies. Might be easily passable, but without further tags it is impossible to know if is hard (or even impossible, ie. vertical ladders for bikes). Not at all sure about horses.
There is even some mapping between sac_scale and mtb:scale, which can also be assumed.