Is engine braking bad in an automatic f150?

It uses more gas. The transmission is OK.

hmmmmm.......so you can use your service brakes and put some wear on the pads/linings. Or you can use engine/transmission for braking by creating engine vacuum but also wearing the valves, valve seals, piston rings, as well as stressing transmission bearings, u-joints, etc. etc.

Think I'd rather service the brakes a bit sooner than rebuild the engine/transmission/driveline.

Wont hurt it at all. My Tundra even has a rocker switch on the shifter for that exact purpose. I can manually shift up OR down.

Been down shifting my Honda minivan for over 100K miles and she still works just fine. Relax

One final note. I never needed to do ANY brake work on the van until it had 90K miles on it. (Obviously the result of slowing down using engine braking)

Yes it's fine.

Well OK, every time you use the transmission, it creates a little more wear on it, so eventually it wears out/ But that's the same for a manual transmission as well.

But manually shifting down to use engine braking is a normal driving technique. Because the engine is on "over run", it wont use more gas, and may in fact save some. Although the revs are higher, the computer can cut the fuel off completely from the injectors as long as you are above a certain rpm. As the revs drop, it has to start adding fuel again to keep it running.

A lot of modern automatic vehicles have a "sport" mode, or even paddle shifters on the wheel to help you do this

At 50 mph, no, not a problem. However, what would be your desire to do such at that speed? Simply wait until you are at a slower speed to disengage the O/D.

It's somewhat counterproductive, since whatever engine-braking effect you get is going to be somewhat offset by a sudden jump in engine speed. Since there's no way anyone is going to tell you that this is cool, I'd advise against it.