Yeah, most big Google services -- including Search -- rely pretty heavily on unknown field retention. Google has been building large services out of microservic

Yeah, most big Google services -- including Search -- rely pretty heavily on unknown field retention. Google has been building large services out of microservices since a decade before anyone ever said the word "microservice". When one service is updated to emit a new field, and another service is updated to consume it, it's important that the feature can then work, without updating all the middlemen.