So many songs would be lost to us if we start censoring retrospectively. Would we start with songs about drugs such as Golden Brown, Mr Brownstone, Back to Black, and a million others? Eminems albums would be straight out of play if we censor songs about violence to women.
I can't think it would be more to the point to keep them and use them as a discussion point about the attitudes towards drugs, violence, slavery, etc.
The only time I think songs should be banned retrospectively is when the singer/songwriter has been convicted of a crime like Gary Glitter. They should not be allowed to collect the royalties.