sushe- I will speculate that Sefardim have different leaders and they didnt institute this ban, and/or that Sefardim lived in a different country than Rabbeinu Gershom.
I highly doubt anyone can make a cherem today like it was done back then.
I believe the expiration date was 500 years from inception which has already passed, but klal yisroel accepted it further. (This was recently mentioned in another thread)