Apple has been supporting "Command+Shift+D" in it's email programs (OSX and IOS) for quite a while. I cannot whether this predates OSX but my intuition tells me no. That the command was/is an homage to earlier email systems that would connect to SMTP. The SMTP protocol expects a Control+D to close the session, as to many Unix programs that accept console input.
(unfortunately it's not consistent... like Blogger for IOS(not an Apple product), iMessage(Apple) and I'm certain many others.