What kind of maintenance do you mean?

A design can evolve over time, but a design document's objective is to document what was going to be built at that time. If something changes, make a new design document. (Similar to blog posts or news articles, they also don't evolve over the years. You write a new one.)

It sounds like what you mean is system documentation, a handbook of sorts, and that's what needs maintenance. But that's different from a design doc.