We were on this one and had to cancel.
Because it is a longer than 10 day cruise, there may be concerns in case someone became very ill, there is a limited amount of hospitals in these tiny towns in Greenland. Or, the hospitals may be near capacity. Also, leaving from NY, which has a high rate of covid. Many things could be a factor. The important thing, the booster can be taken anytime before sailing.
We are double boosted, so it would not have been a problem.
Princess has stated many times, highly recommend as some countries may require the booster or rules could rapidly change.