The Zip+4 last four digits align to delivery zones. It can be trivially constructed from the complete address now that we have reliable digital mapping systems, and in fact this is what happens internally in the postal system.
It is not required and will likely never be required to provide a 9 digit ZIP for reliable delivery. It may, and does sometimes, impact speed of delivery due to sorting/distribution rounds.
> It is not required and will likely never be required to provide a 9 digit ZIP for reliable delivery.
That depends on who you are.
If you are a regular person, then yes, 5 digits is sufficient. But if you are a sender of presorted commercial bulk mail (which is discounted from first class), you may actually be required to provide a 5 + 4 + 2 = 11 digit ZIP.
That little barcode the post office prints on your letters is actually just the 11 digit zip. The final two digits are the last two digits of the house number. So "123 Any Street, Anytown FL, 45678" the final two digits of the zip would be 23.
It is not required and will likely never be required to provide a 9 digit ZIP for reliable delivery. It may, and does sometimes, impact speed of delivery due to sorting/distribution rounds.