Micron NOR unused pins have pull-ups. Any danger just leaving them there?
These pins can have any number of uses. Sometimes they enable internal test modes or other funky connections and can cause malfunctioning of the device 

In one possible case given the information provided, they may be binning dual-die devices to single die when there is a failure or defect in one die instead of scrapping all together. This may leave some internal structure/electrical connections in tact, but this may not be consistent if they still run dedicated single-die production, the pins are reserved to indicate the potential for oddness. 

The key is that


  Reserved by Micron for future device functionality and enhancement. Recommend that these be left floating. May be connected internally, but external connections will not affect operation.


It means they are giving you the go-ahead that you will not break the IC with external connections as long as they respect the absolute maximum ratings which apply to all the pins, but that doesn't mean that external connections may not cause electrical conflicts/shorts/additional current draw in certain drop-in applications. 

Likely leaving a weak pull-up will have no long term effect as you have verified/tested.