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.

Interesting.
What does it do when you shine the light on the card numbers?

Is the squiggly line an antenna?

I recently took an x-ray of a card, might be interesting to some of you as well:

Peak mildly interesting, bravo

So metal

Doing that can reveal a whole bunch of interesting patterns. I was repairing something on a circuit board once and was trying to figure out the wiring. Trying to get more light, I shone my phone's flashlight at it. At some point I suddenly realized that the circuit board lets a bit of light through. I realized that, at least for some of the board, I could shine a flashlight onto the other side and see pretty exactly how the circuit was laid out. It also looked really, really cool.