Is There a Constraint on a Column?

The constraint that only applies to future records and leaving it like that permanently might not be the greatest solution. An outside person, not familiar with how all that was done, might notice that they can't insert anything new that has a noll in this column yet there are old records. That seems like a bad thing, open yourself up to confusion. You want your data base to express the constraints around the data that it's storing in a way that is helpful to the reader.