I read a lot of history concerning battles and the casualties are almost always in the thousands after each engagement. It's probably no less than 100 million that fought and died in these historic conflicts. Morbid as this may sound, that's a lot to clean up. I do know that a lot of looters would come in and take whatever they find such as gold, jewelry, and even teeth, but armor was always heavy to carry too much of.
So what happened to all that equipment? I can't find any evidence on the internet.
I can’t speak to the entirety of history, but I think the battle of Visby is a great example: the defenders were using mostly obsolete gear, so they were just left in it. They were either buried in mass graves, or left in the field, wearing the armors of their fathers and forefathers because it wasn’t worth the effort to strip it. I imagine this happened often.