Minimum Clock Source for the I2C Driver
I²C defines a maximum speed (actually, a different one for each mode), but the purpose of that is only to ensure that the electrical signals can be handled by the circuit without degradation.

The speed of the transmission itself is not fixed, but can be defined by the clock signal sent by the master (optionally slowed down by the slave through clock stretching), so the master can choose to use a slower speed, and can even use different lengths for different bits.

The I²C specification does not define a minimum speed, so in theory, you could use any clock source, or even clock out the bits by hand with a (debounced) button.
In practice, SMBus (which is derived from, but not identical to, I²C) defines a timeout of 35 ms for a low clock signal, and some I²C implementations might have a similar timeout in order to detect hung devices.
So for those cases, a minimum speed of at least 50 Hz (0.05 kHz) might be advisable.

The code you've shown does not put an absolute limit on the I²C clock; it just checks that it is actually possible to derive the chosen I²C clock from the chosen clock source (with a divider between 1 and 256 if I'm reading it correctly). So you can use any speed, as long as you have a matching clock source.