Reading on the issue: https://github.com/gravitystorm/openstreetmap-carto/issues/180 It appears the biggest reason it's not rendered is about styling (whether people want to see that much detail). The fact that it isn't approved is not as important (lots of OSM features never went through the process) However reading there and on the forum, there seems to at least be fair consusus that area:highway=* is for linear features and area=yes is for omnidirectional features.