What is the "Leftover shortcut" item when uninstalling a moved app?
Short answer: It's the shortcut a moved app left in Applications. Uninstall removes it with the app so nothing is left pointing at a missing app; it takes almost no space.
When you uninstall an app you had moved, its shortcut is still sitting in the Applications folder pointing at the app on your fast drive. Storage Studio lists it as Leftover shortcut so it's removed along with the app, rather than left behind pointing at nothing.
It takes almost no space, so it shows the word "shortcut" instead of a size.