So I found some stuff in SMW

[Ton]

We need some kind of strike force that looks for unused stuff in games.

I'd join it!

Those cape sprites are used in the game, just not for the death sprites. The varieties of Super Koopa that run along the ground before taking off use them.
Here's a screenshot from Donut Plains 1 for proof:


The Super Mario Bros. subtitle also has a number 4, which exists in the data as well:


This can be seen on the Japanese box:
Link

Hey, you finally joined! Good to see you here.