Change the status of ADSP (RFC 5617) to Historic
status-change-adsp-rfc5617-to-historic-00

ADSP has garnered almost no deployment and use in the 4 years since its
advancement to IETF Proposed Standard.  While there are implementations in
code, there is very little deployment and no evidence of the benefits that were
expected when the standard was written.

There is, however, evidence of harm caused by incorrect configuration and by
inappropriate use.  There have, for example, been real cases where a high-value
domain published an ADSP record of "discardable", but allowed users on their
domain to subscribe to mailing lists.  When posts from those users were sent to
other domains that checked ADSP, those subscriber domains rejected the
messages, resulting in forced unsubscribes from mailman (due to bounces) for
the unsuspecting subscribers.

Assurances that are provided by ADSP are generally obtained out of band in the
real Internet, and not through ADSP.  Current deployment of ADSP is not
recommended.