I don't believe it was ever in the regulation including the official staff commentary. When the FRB published a final rule on Regulation B in the 3/18/03 issue of the Federal Register, they mentioned privacy in the supplementary information section. Here's what they said.
Many commenters were concerned
about the co-applicant’s or guarantor’s
privacy when the reasons for adverse
action pertaining to creditworthiness are
given to the primary applicant. When a
person agrees to be a co-applicant,
guarantor, or similar party, however,
there is (or should be) a general
understanding that information will be
shared. Accordingly, the rule has been
adopted as proposed.
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Opinions expressed are my own and not necessarily those of my employer. They are not legal advice.