Protobuf serializes structured data into a binary format for maximum efficiency/performance.
OpenDDL seems to be focused on a text-based format that is human-readable and editable.
They seem similar in that they both provide a means to model data using a type system, but different in that one is focused on efficiency/performance while the other is focused on interoperable but readable/editable data.
It's related to OpenGEX (Open Game Engine Exchange format), and coming out of the gamedev world. It's more for interchanging assets between authoring tools and game engines, less for sending message-like data over a wire. It's also not a new thing, has been around for a decade at least.
