Not aware of any actual grpc websites but given grpc-web has 6.5k stars on github something must be out there.
Google's websites frequently use binary-like formats where json is just an array of values with no properties, and most of these values are numbers. See for example Gmail.
That's likely just an encoding into JSON of protobuf messages. The typical wire format contains a sequence of field tag followed by contents. They can just make it an even-sized array.
