Electrical Engineer here. Pretty sure the slit is to break up the metal in the card so it won't interfere with NFC.
If you look at an NFC antenna, it's a big coil of wire. It's energized by an oscillating magnetic field that passes through it. Faraday's Law dictates that when a magnetic field oscillates, an electric field will form around it. This electric field generates a current in the antenna that powers the NFC chip.
In a metal card, the current will flow in the card instead and draw energy away from the NFC chip. This slit blocks that current flow so that the current flows in the antenna instead.