What is significant about the fact that the past is referred to as the Unmentionable Times? How does the past probably compare with the period in which the novella takes place?

This is a dystopia. The authorities attempt to dismiss the past as a time of suffering and injustice in order to validate the present. The present time is so dismal that we know the past would have been better in so many ways. There are no details of the past given, the past is merely "unmentionable." Eventually all the people will know is a past present and future under the same authorities.