Not running down Signal or any other e2ee client out there. But there are in fact ways to prevent the recipient from retaining and, at least via the device they are using to receive the messages, copying the data within messages. Several banking apps actually include such measures. And perhaps even having a Mission Impossible-esque self destruct on the message where it re-encrypts and then deletes the messages after they have been viewed is a viable way to prevent retention. It would not prevent somebody from using another device to snap a picture of the information in the message, but it would definitely cut down on people's ability to easily leak private conversations that are intended to stay that way.