Be'er Hagolah at the end of Yoreh Deah 334 says that one of the things that are considered Cherem D'Rabeinu Gersom is not to embarrass a Baal Teshuva regarding his previous lifestyle.

Yet when you look in Bava Metzia 58b the Gemara counts it as Onaas Devarim.

Why then was there a need for this Cherem?