The deposit of faith does not just consist of doctrines that have had a specific verbal formula infallibly defined. General doctrines of faith and morals that have been taught always and everywhere in the Church are part of Sacred Tradition and must be held, as concepts, by the faithful. The sinfulness of abortion has always and everywhere been taught in the Church.
And the mandate not to do abortions is even older than the Catholic Church, since it is in the Hippocratic Oath.