The site has been vacant for a long time with no current use, and agreed that "brownfield" in the non-OSM context usually means polluted land but it does seem the OSM schema for the "brownfield" tag is different and is intended to map sites that are vacant or that once had a building: https://wiki.openstreetmap.org/wiki/Tag:landuse%3Dbrownfield so it seems that this tag would be appropriate within the OSM brownfield schema. I have made the change for now.