If routers are telling pedestrians to walk in the traffic lanes of a motorway, as opposed to a shoulder or sidewalk, then they’re probably in deep trouble whether they go with or against the flow of traffic.
In the Americas, most secure international border crossings have one-way sidewalks leading up to the checkpoint. Many public airports also have security checkpoints and exit lanes that rely on a combination of armed guards, security cameras, timed audio cues (“Keep moving!”), and automated gates to ensure that passengers proceed expeditiously in one direction. I don’t think we need to be concerned about the possibility of moonwalking through a security line.
My first use of oneway=yes on a highway=footway was this footpath in a park in Texas. After visiting for a second time, I noticed the small signs requiring pedestrians to walk clockwise around the lake, stay off the grass, and avoid feeding the wildlife. This was a month after the initial Pokémon Go release, so the park was swarming with teens fixated on that game, roundly ignoring all three regulations. Nevertheless, the oneway=yes tag was valid information about the path. Valhalla routes clockwise around the lake as I’d expect, but apparently this is surprising to some here.
The standard 
and 
signs are only for vehicular one-way restrictions, so they aren’t used in the cases I described. For that matter, these signs are also disallowed on one-way streets that have contraflow bike lanes, in favor of Do Not Enter, Wrong Way, and No Left Turn signs.
At least in the routing engines I’m familiar with, this is actually rather natural. For example, OSRM has separate profiles for walking, cycling, and driving. A separate routing graph is computed for each, so the mere fact that a road is represented by an edge in the routing graph means that it’s relevant to pedestrians. At this point, the router has no context about other modes of transportation, but if the edge has a one-way flag based on oneway=yes, then the router could feasibly assume it applies to pedestrians.
As a matter of fact, OSRM’s default foot profile only honors oneway:foot=yes and ignores oneway=yes:
This would seem like a slam dunk in favor of your proposal, but the underlying reason isn’t what you think. Since the very early days, OSRM has taken the stance that both pedestrians and cyclists should ignore one-way restrictions unless there’s a more specific restriction (e.g., oneway:foot=yes). After all, the street may be restricted to one-way traffic, but this doesn’t stop a pedestrian from walking contraflow on the shoulder or sidewalk, which is assumed to allow two-way movement.
Likewise, OSRM tells cyclists to walk their bikes against the flow of a one-way street, again assuming a sidewalk or sidepath. Confusingly, OSRM doesn’t yet honor one-way overrides on sidewalks or bike lanes, though it did add a heuristic that prevents this contraflow movement if there’s a bike lane. Perhaps it should also account for sidewalk=use_sidepath and sidewalk=separate.
This is a very reasonable strategy, but it seems to me like a two-step strategy for a regional tag cleanup campaign rather than a global tagging proposal. The proposal states a specific known scenario in which oneway=yes has been skunked, in a particular country, so why not use this strategy to clean up the tags, rather than forcing the rest of the world to reaffirm oneway=yes really=yes on footpaths that aren’t such a known quantity? A little temporary redundancy in one country instead of permanent redundancy in every country.
To me, the concept is simple. oneway=* on a roadway applies to everyone in the street, but the effect on the shoulder or sidewalk is undefined. On the other hand, oneway=* on a footpath applies to everyone on the path, because footpaths are for feet. If an occasional maintenance vehicle is also allowed on the path and can go in either direction, there’s a oneway:motor_vehicle=* key for that. If bicycles are allowed on the path and can go in either direction, there’s a oneway:bicycle=* key for that. This is what people mean when they say that pedestrians have “priority” on a footway, right? 