Using ATtiny ADC with a system clock of 32.768 kHz
The minimal frequency is because of the sample and hold capacitor leakage. When conversion is too slow, the stored voltage can leak more than 1LSB and this way to compromise the conversion. 

This way you will get lower accuracy than specified.

Notice that the minimal frequency is defined for the worst case - higher temperature, extremal (lowest or highest) power voltage, maybe external radiation. This way, the described problems can happen later during the device work.

So, the conclusion is that if less accuracy is not a problem - go with it. But if you want to use the full accuracy of the ADC, the minimal frequency should be respected.