ISAAGNY’s (Independent School Admission Association of Greater New York’s) website clearly states:
First Choice Letters
ISAAGNY does not encourage formal expressions of first choice.
ISAAGNY may not now officially “encourage” first choice letters, but many individual Manhattan, Brooklyn, and Riverdale private schools still do. This year, as always, first choice letters or verbal commitments are often de rigueur. Private schools want to predict their yield–the number of accepted students who will actually enroll. First choice letters give them a key insight.