Rather than compressing a judgment about reality into a single boolean, it seems more natural to break it down into smaller observations and evidence, then derive conclusions from them. Users wouldn't need to touch the ontology directly; they'd just answer small, concrete questions like "Is there a sidewalk here?"
FWIW this is exactly how it works on OSM, and probably why OP is complaining people were being pedantic by telling him he "shouldn't mark a street as not walkable because there is no official sign that prohibits walking".
"Would an ordinary person want to walk here?" is pretty subjective. "Are people legally prohibited from walking here?" is pretty objective. Generally only the latter would be recorded in OSM.
