What was the first year that impala come with disc brakes?

The Impala didn't have disc brakes on all four wheels from the very beginning. The introduction of disc brakes on Impalas was a gradual process, varying by model year and even sometimes the specific trim level. There wasn't a single year where *all* Impalas suddenly got them.

To get a precise answer, you'd need to specify which Impala model (full-size, SS, etc.) and whether you mean front disc brakes or four-wheel disc brakes. Generally speaking, front disc brakes started appearing on Impalas earlier than four-wheel disc brakes. You'd need to consult a detailed Chevrolet Impala model history for the specific year they became standard or an option on your chosen model.