Would it be possible to build a device that allows you to walk backwards?

I don’t know, but I think you can name it the ”Moon walker”.

When I ride, I have rear view mirrors on my helmet to be away of my surrounding.
You could utilize a few to view as you walk. It does relieve pressure on your knees.
I also do this on a treadmill from time to time. Your glutes will also an get enhanced workout.
You may even teach your brain a few new tricks as the process will do a neuro change by doing this.
Great Idea.

I think what would be difficult about walking backwards out in “the wild” is that you can’t see what’s on the ground in front of you, uneven terrain, curbs, etc. In the physical therapy session, you know that the floor is flat and so you are confident you won’t trip. Outside, you don’t know if there are hills or mounts, or curbs or steps or what. Even with a rear view mirror, you can’t see the ground.

Something that occurred to me is to try walking backward on a treadmill According to the article, is should be possible to do it safely

You mean like the Backwards Man?

Yes. Be sure to name if after yourself so you will get full credit.

I know a guy who build a backwards pedaling bike for his disabled neighbor. The disability prevented forward pedaling motion.