Its an interesting theory and there are a few doctors who support it. Of course the same could be said for any number of quack devices and therapies. If the earth has a negative charge it must be in relation to something with a positive or less negative charge. When something is ionized, it gains a positive charge by losing electrons. Do we have a positive charge possibly from digestion etc? It certainly could be the case.
There are a few holes in the theory, they say walking on sand or concrete will do it. But sand is not conductive to any degree, concrete may be slightly conductive depending. They say leather shoes allow the charge to flow but leather is insulating and people wear socks. If we are talking about a high voltage low current like a static charge then it might be possible. Its easy enough to try I guess
I went outside and walked around barefoot for a few minutes. I felt better afterward, I guess that proves it, lol. Obvious placebo effect or just the fact its more pleasant out than in. I have no easy source of ground in the house so my experimenting will be limited. I could bring a wire in but I'd like to see a few reports from our members first before I bother with it. Walking outside is easy, its always nice at the beach so testing won't be unpleasant but you probably need a bunch of hours on it to do anything perhaps