Don't know if it is official. My own experience was that my RTW itin was on hold for 60 day initially. Days later, I was informed I had 24 days to buy. I asked why. They said one of the airlines (RJ, in this case), had put in that new restriction. So, I would not consider 21-day hold as the golden standard. In fact, you should ask RTW DESK what is the LONGEST holding period allowed on your particularly itin.