DNU pin may be internally connected to the die. It might be used for test mode entry and hence, these pins have to be left unconnected/floating. For RFU pin, no internal signal is currently connected to the die but there is potential future use of the connector for a signal hence we recommended this pin to be left unconnected/floating.
In the unlikely event that DNU/RFU becomes Hi or Low due to contact with the neighboring pin, etc. As a behavior of Flash, we have received inquiries about what kind of things can be assumed
Is it possible to answer the situation in such a case?
ex1. DNU -> Hi or Low
ex2. RFU -> Hi or Low
As Nada-san already mentioned, RFU pins are not connected internally to the die. So, it is OK to make RFU pins either HIGH or LOW. DNU pins are connected internally. But, any DNU related functions will be inactive if the signal level is at VIL (Refer Table 2. Signal List in datasheet), also there is an internal pull down resistor on DNU pins. So, there will not be any problem if you make DNU pins LOW.
If you make any of the DNU pins HIGH, then the device may not function properly.
Thanks and Regards,