Automatic transmissions avoid engine braking. If you want it, you need to choose it manually.
And personally I never do for the simple reason that brakes are a lot cheaper than transmissions. And I rarely descend a hill long enough and steep enough that brake heating is a serious problem. (And when I do I usually just slow down, and stay slow.)
I downshift on hills because that keeps more of my stopping power ready to use. Brakes heat up when you use them continuously, reducing their effectiveness.
And personally I never do for the simple reason that brakes are a lot cheaper than transmissions. And I rarely descend a hill long enough and steep enough that brake heating is a serious problem. (And when I do I usually just slow down, and stay slow.)