I already added them again, don't worry. Some of these tags are set only to prevent being asked again to other StreetComplete users (like name:signed=no, opening_hours:signed=no, etc.). Many countries have a rule in their traffic code that says that if a bridge is higher than 5 m, there is no need for a sign, so maxheight=default or maxheight:signed=no then means it's higher than 5 m. maxspeed:type=BG:urban is another reference to the traffic law of a country: it used to mean 60 km/h until Bulgaria changed its maxspeed inside towns to 50 km/h. Thanks to tagging it like that, we don't need to change thousands of maxspeed tags when the law is changed.