if it is an "easy to produce or consume in language X" it does not mean it is canonical - it means that language X has an extension that allows to do so. Is there a place in protobuf spec or documentation mentioning this to be a part of the protocol?

Language X is at least C++ (as linked), Java https://developers.google.com/protocol-buffers/docs/referenc... , and Python https://github.com/protocolbuffers/protobuf/blob/master/pyth...

I think the Protobuf spec focuses on the binary serialization - the text format and JSON representations are not related to that at all, of course.