Dunno if anyone will ever find this bit of info helpful, but... if anyone wants to whitelist the shells (DA-DD), you'll have to whitelist koopas as well — because "DA-DD aren't shells, they are run-once sprites that spawn a koopa in a stunned state" (courtesy of The_Biob).
So if you want to whitelist the blue koopa shell (DC), you'll also have to whitelist 06 (normal blue koopa).
Works as intended, so, accepted. However, there is an issue with killing the springboard sprite: it seems to affect Mario's Y position and sends him off the top of the screen. I couldn't recreate this behavior with any other sprite, though, so I can only assume it is one of the springboard's janky behaviors. Therefore, I changed the file to have the springboard non-killable by default, and added a note in the readme.