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.
Opinions expressed are my own and not necessarily those of my employer. They are not legal advice.