It seems to me that it would be in the spirit of Dorico to do the subtraction in the last bar of a piece or before a new time signature/pickup automatically. Of course, whether this is done at all depends on the style and period of the music, but it would be a helpful option to have.
musicmaven:
There is a big difference, however, because a final barline will not allow anymore measures or beats to appear. It is final. But the concept of delineating the number of beats in a measure can be achieved with placement of barlines. Caveat emptor … users might want to be careful to follow traditional rules of showing time signatures (in traditional musical contexts).
In addition to what musicmaven said, not every score that starts with a pickup measure has a shortened final measure. I have seen a few scores in my time where this happens. It is rare, but not unheard of.