It's indeed a shame that protobuf has so many bit patterns available for the first byte (lowermost 3 bits can't be 6 or 7, and 4 cannot be also used alone), and

It's indeed a shame that protobuf has so many bit patterns available for the first byte (lowermost 3 bits can't be 6 or 7, and 4 cannot be also used alone), and some of them could have been used for a unified length-delimited framing format.