From the book Music Notation by Mark McGrain, published by Berklee Press (Berklee College of Music), in print since at least 1986, page 137:
“The abbreviation “8va. bassa” or “8vb.”, placed below the staff indicates that the passage is to sound one octave lower than written.”
Given Berklee’s worldwide influence in the fields of jazz, commercial music, TV and film music, and popular music, I think it’s a mistake to dismiss 8vb as incorrect or “wrong”. Please at least offer it as an option in Dorico.