if everything does hinge on the status of the port, they may give the cruise lines typical construction updates (“Will / Won’t be done by X date”). So, NCL may know only a few weeks out each time.
From what I’ve seen on other threads, some people have gotten an email prior to sailing. I wonder if NCL figures at least some advance notice is enough for people to cancel third party excursions without penalty. So if Isafjordur is at the end of your itinerary, knowing at embarkation is enough. If it’s at the start, they let you know in advance. I’ll see if we hear anything from NCL before our July 6 embarkation.
Agree with all who said that even if the circumstances are beyond their control, NCL (and maybe all cruise lines) need to improve their transparency & communication.