I'm not aware of any discussion about it. I think highway=pedestrian & area=yes is an older method. It's used 230k times, about the same as area:highway=*, and area:highway=pedestrian is used only 7.4k times. FWIW Carto renders only highway+area.

I don't think it makes sense to have both and would support deprecation of highway+area, but I wouldn't retag existing features before it's officially done.