Q:

What ist this component on early network cards with a slit in its package?

Early PC network cards (10BASE2, with a BNC connector) often had a two-pin component, typically yellow, with a slit in its package, close to the connector (image source):

What is this component, and what is the purpose of the slit?

A:

It has a designator of C38 which means it is a high voltage capacitor with a spark gap.
Typically the coaxial cable interface is isolated from the PC chassis ground, with the exception that the cable shield is connected to PC chassis via a 1 Mohm resistor in parallel with a 10nF high voltagre capacitor. The capacitor voltage rating typically is 1-2 kilovolts. The 10nF capacitor slows down the AC transients and the 1Mohm slowly discharges the charges that have accumulated. In the event of high surge voltage, it is discharged to PC chassis via the spark gap.

