You are probably thinking of draft-ietf-822ext-mime-conf-01.txt,
4. MIME Conformance
A mail user agent that is MIME-conformant MUST:
(2) Recognize the Content-Transfer-Encoding header field
and decode all received data encoded with either the
quoted-printable or base64 implementations. Any non-
7-bit data that is sent without encoding must be
properly labelled with a content-transfer-encoding of
8bit or binary, as appropriate. If the underlying
transport does not support 8bit or binary (as SMTP
[RFC821] does not), the sender is required to both
encode and label data using an appropriate Content-
Transfer-Encoding such as quoted-printable or base64.
It is hard to see how this means that support of ESMTP is required.
I think, however, that the prose *should* say that the behaviour of
one certain gateway, whihc rejected any message containing
"Content-transfer-encoding: 7bit", is completely broken.
Even display of messages containing "Content-transfer-encoding: 8bit"
and arriving over unextended SMTP (the 7-and-sometimes-8-bit transport)
should be displayed, I think.