As far as I can tell, the field is always empty. Is this a legacy thing or is there an undocumented way to turn it on?
It is from the output of our public transit module: graphhopper/reader-gtfs at master · graphhopper/graphhopper · GitHub
We should probably remove this for unrelated endpoints. And see here on how to extract this information per “leg” for these endpoints: Leg-based information - GraphHopper Directions API