of Historic is one that the technology defined by the RFC
is no longer RECOMMENDED
for use, which differs from "OBSOLETE
" header in a replacement RFC
For example, RFC 821 (SMTP) itself is OBSOLETE by various newer RFCs, but SMTP itself is still "current technology", so it is not in "Historic" status.
On the other hand, since BGP version 4 has entirely superseded earlier BGP versions, the RFCs describing those earlier versions (e.g. RFC 1267) have been designated Historic.
There might be more information for this subject on one of the following: