The USPS loses far more money by other schemes, but the the 9 cent cheating was obvious to the public and got ink from the press, so an easy target for the USPS to address. Eliminating the "0" was a small visual changes to the design, but the real change was to cease tagging low-denomination stamps of 10 cents and less beginning in about 1995.
Increased automation made it far too easy to get any tagged stamp through the system. It is even easier to get underpaid mail through the system today with machines taking a bin of mixed mail, facing it, (maybe) canceling it, and after more machines and transporation and more machines having it eventually arrive to the carrier in delivery order sequence - all without ever being seen by a person.
Here is another example from 1993. It bears two inverted 01 kestrels appearing to generously overpay the 19 card rate by 1 cent, yet costing the sender only 2 cents. The stamp is tagged and thus easily machine-faced and canceled. In reality, it is an oversized card at 5.5" x 8", so requiring the 29 cent first class letter rate at that time, so an underpayment of 27 cents. Successfully mailed for 2 cents.
