I’ve dreamt of something like this too, but I could imagine it being quite difficult to support well.
Unless the visibility graph accounts for things like walls or long benches cutting across a pedestrian plaza (as PlazaRoute might), the result would be something of an approximation. Routing over open space would increase the need to account for barriers that aren’t as easy to connect to the routing graph, such as waterways, as well as elevation changes that might have to come from an elevation model rather than OSM.
The documented notion of default access restrictions on highway=* ways already suffers from poor software support, let alone default access restrictions on landcover areas. Are we going to start blanketing these areas with explicit access tags, just in case, like we often do on roadways?
One of the assumptions of these routing graph enhancements is that the entrances and exits are modeled explicitly as connections between pathways and pedestrian areas. But mappers really don’t like to connect landuse areas to roadways and pathways, let alone landcover areas.
Defining highway=pedestrian based on these exceptions is really unintuitive. It would be like defining highway=motorway based on the kind of vehicles banned from it. A more intuitive distinction would be highway=pedestrian areas for pedestrian plazas and highway=pedestrian ways for pedestrian streets (pedestrian malls), but highway=footway areas for any other space built for undirected foot traffic. The term “pedestrian street” makes it pretty obvious that the way functions like an ordinary street except for the kind of traffic that normally uses it. That can explain not only the access exceptions but also the name in many cases.