SpaceX

It's mostly steel. Nothing toxic, not even kerosene or solid propellant.

That is the fate of pretty much every rocket booster with the exception of Falcon 9. Starship is designed to be a fully reusable launch system (booster and upper stage) and these tests are stepping stones towards that goal.

I agree that its not great to dump items in the ocean. But given that it is rather hard to predict exactly where it will drop, and how fast if would sink after hitting the ocean surface with significant velocity, it would take an army of boats to recover anything.

For rockets i feel you have two large impacts to the environment, the production cost and the burned propellant. For starship the construction does not require too much harmful material i geuss. And the fuel is methane and oxygen. Methane will convert to CO2, but it does not have any nitrogen. Not as good as hydrogen + oxygen in the combustion product which only leaves water.

Looking at environmental impact is always hard, because you need to look at the whole lifecycle. Which frankly i am too lazy to compute here. Perhaps they have performed some research into it required by some legislation.