[ietf-dkim] 8bit downgrades

Can anyone remember why there's a SHOULD for the downgrade to 7-bit in RFC4871 
Section 5.3, rather than a MUST?  The likelihood of breakage is so high when 
sending 8-bit data that DKIM almost becomes pointless without the upgrade.

Not advocating for this to be changed in -bis (yet), but someone's asking me 
for the history behind that decision.

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