water can get into the slice and into the layers of the tire and then start to corrode the metal cords weakening the whole thing. also hard to tell how much of the cords are damaged with a slice like that. I wonder what caused it.

As someone who works in the tire industry, I'd say that tire is dead, unfortunately. I'd recommend replacing as soon as possible and not to attempt driving on it much, as you can cause unseen damage to the inside and compromise the integrity further.