IIRC, The method for sending messages using a Patronus was developed in canon by either DA or OoTP, and is not common knowledge. In MoR, Harry develops it himself but it presumably has been used in the war games by now.
ETA: Chapter 28, it’s mentioned that Minerva’s cat patronus contacted Harry, so that’s probably where he got the idea.
IIRC, The method for sending messages using a Patronus was developed in canon by either DA or OoTP, and is not common knowledge. In MoR, Harry develops it himself but it presumably has been used in the war games by now.
ETA: Chapter 28, it’s mentioned that Minerva’s cat patronus contacted Harry, so that’s probably where he got the idea.
not the DA, but I think OotP