Deprecate rf= and maybe fo= tag

[johnl@…]

The only valid value for rf= is rf=afrf. It appears unlikely that there will ever be a report format other than ARF, so deprecate this tag.

I do not know how widely the fo= tag is implemented, asking for more failure reports. Seeing how few reporters produce any failure reports at all this might go, too.

[todd.herr@…]

Regarding the fo= tag usage, the following was reported in ticket #4:

A survey of nearly 74,000 DMARC records that Valimail has scanned found that just 316 (0.42%) were using the colon-separated list as a value to their 'fo' tag.

Other interesting numbers from the 73,917 domains...

5,906 with a value of '1'
  517 with a value of '0'
   74 with a value of 's'
   11 with a value of 'd'

  128 with a value of '1:d:s'
  115 with a value of '0:1:d:s'

[todd.herr@…]

Proposed text to deprecate rf=. Left fo= alone.

Pushed to github and merged to main branch.

[todd.herr@…]

Consensus from 27 May 2021 Interim (​https://datatracker.ietf.org/doc/minutes-interim-2021-dmarc-01-202105270900/) and the working group mailing list (​https://mailarchive.ietf.org/arch/msg/dmarc/psW6Ad8FRZ1ynRlskQsrQSTpVsQ/) is to remove the rf tag from the base spec.