The BorrowerUnwantedField preference lets you hide fields from the patron add form, but in the OPAC those fields still show for patrons to update their profile if you allow them to with OPACPatronDetails. The BorrowerUnwantedField should also hide these fields in the OPAC.
This is fixed with the introduction of the PatronSelfRegistrationBorrowerUnwantedField preference in 3.12