Does it make more sense to just say something like "Received a message encrypted using an old version of TextSecure that is no longer supported. Please ask the sender to upgrade to the most recent version and resend the message."? Ideally we can avoid words like "protocol" that are really only important for those who want a deep understanding of the inner workings. The problem is an old version of TextSecure, the solution is to upgrade, the technical reason of it using a different protocol isn't an empowering bit of knowledge in this case.