Thank you! I've wanted to try a '3D model with a 2D projected viewport as the main game' thing for a while and this jam gave me the perfect excuse to try it.
A mix of things!
Some things like the character sprites have the shadow/ghosting baked into the sprite. But for the projectiles and other screen elements I set up a custom script that automatically added the ghost/shadow effect. (Ideally, every sprite would use this script because it's more customizable, but because of the jam time limit I decided to brute force it near the end of development via the first method I mentioned, like with the character sprites haha) It's deceptively simple. (If this is what you're talking about)