> Protobuf is another library to handle cross-language serialization, but wouldn’t offer us the flexibility to add all the other features we now have — plus, it was a binary serialization when I last used it.

Protobuf has a well-defined text format, which is the top search result for "protobuf text" [1]. Proto3 also has a clearly defined set of rules for supporting JSON.

[1] https://protobuf.dev/reference/protobuf/textformat-spec/