As per this exchange, <author> also needs an additional varint to indicate the feed type.

There was also this idea for an additional varint in cypherlinks to messages, but I'm not as convinced anymore. Messages should be as self-describing as possible, including their encoding. I'd rather keep an encoding indicator in the message metadata (talking about the metadata improvements now, not legacy metadata), and distinguish between legacy messages and self-describing new messages via the hash indicator ("hash tag"?). So unless anyone complains and I don't change my mind on my own, that's the road I'll explore further.