https://www.openstreetmap.org/way/1259166980/history has maxspeed:source = sign

is it intentionally used instead of source:maxspeed=sign ? See https://wiki.openstreetmap.org/wiki/Tag%3Asource%3Amaxspeed%3Dsign

maxspeed:type is also sometimes used as a key for such data, see https://wiki.openstreetmap.org/wiki/Key:maxspeed:type