This is indeed an oversight. Too bad you did not tell us about it a few days earlier... and I'm kind of surprised this does not happen at the RFC editor side automatically.
Part of the problem is that RFC 1123 does not state that it updates RFC 821, although RFC 2821 does state that it obsoletes 821, 974, 1869 and updates 1123.
Anyway, as a conclusion I'm suggesting we send an errata entry to the RFC Editor that shows the 821 update and 2.6 new situation.
I think we also need to check that the ABNF in RFC 4282 is compatible with RFC 2821 ABNF (Section 4.1.2).
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