https://datatracker.ietf.org/doc/html/rfc976 (Feb 1986, but it mostly sets out as standards the practices that are already widely in place).
Indeed, RFC977 (1986) quotes a 1985 message from someone on unitek (a uucp node) requesting a reply by email (while the word "mail" was more often used before this):
Date: 25 Sep 85 23:51:52 GMT Reply-To: honman@unitek.UUCP (Hon-Man Wong) Distribution: net.all Organization: Unitek Technologies Corporation Lines: 12 ... Please reply by E-mail. Thanks in advance.
https://datatracker.ietf.org/doc/html/rfc976 (Feb 1986, but it mostly sets out as standards the practices that are already widely in place).
Indeed, RFC977 (1986) quotes a 1985 message from someone on unitek (a uucp node) requesting a reply by email (while the word "mail" was more often used before this):
So I'm not really sure as to the source of your confusion.