A rather extreme case of oneway=alternating perhaps. The documentation for lanes=* already advises the use of oneway=alternating or oneway=reversible in conjunction with lanes=1, giving the following example that lacks a sign regarding directionality:
Sometimes directionality is evident based on width and adjacent barriers or hazards, in which case oneway=* describes a practical limit rather than a legal restriction. This will probably be a difficult reality to accept for anyone who clings to the idea that access tags never have anything to do with practical usability:
On the other hand, oneway=alternating can be a legal restriction too. California sometimes posts the following sign on narrow two-way bridges:
The sign doesn’t prohibit cars from the bridge, and it doesn’t mean that a road crew emerges to erase the yellow centerline as soon as a truck approaches, then repaints the centerline as soon as it finishes crossing the bridge. Rather, the bridge becomes oneway=alternating for everyone in the presence of a truck. The truck driver must drive astride the centerline while cars, cyclists, and pedestrians wait for it on the other side. This situation is currently documented as a big question mark.
The cave you visited would have no need for such a conditional restriction, but imagine you come upon the bucolic alley shown in the footway=alley documentation and so does someone pushing a stroller. Suddenly the alley has effectively transformed into an overtaking=no situation, with either oneway=alternating or foot=yes back=against_wall.
Fortunately, we’re mapping for route planning and navigation guidance, not writing the script for a sitcom in OSM English. 