Super I/O is dead and gone for like well decades.

For Intel system super I/O was eaten by the ICH which initially kept compatibility with most Super I/O functions.

By ICH 6 or 7 which is around the mid 2000's most of those legacy features were all gone.

By 2008-9 the ICH was also gone and replaced by the PCH which doesn't have almost any compatibility or features with the Super I/O architecture.

What is the nuvoton superio chip on my ASRock z97 then?

Most like the same things the rest of them do like SMBus, temperature sensors, fan control and voltage monitoring. They also do the GPIO for various other features like resetting the bios or switching between different BIOS flash chips, more recently the LED controls on the newer motherboards and the error code segment display that is now used instead of the POST beeping sounds so you can actually know wtf is going on :)

Super I/O is not dead. It has the PS/2 keyboard and mouse ports for example.