Hello,
From the data sheet, it states that the device can be used as a level shifter from 1.6v to 10v.
What it does *NOT* say is that it can be used as a level shifter from 1.6v to 12v.
What this tells me is that it can probably work up to 12v as a true max but for reliability they quote 10v. They cant say 12v because that is the exact point where the leakage current starts to become too great with various other spec limits also in place, and although it still works with the majority of the packages tested, the number of failures at 12v exceeded some lower limit set as a threshold for determining what the max voltage should be. Thus they rate it slightly under their threshold point and know that this is a good practice from experience.
So going over 10v means that you risk that some units will fail (even though others may not), so it becomes more of a risk that there may be a failure much sooner than expected.
We see these kinds of ratings more on data sheets that actually do show the maximum ratings too. For example, i would not be surprised to see something like "12v absolute maximum" in a revised data sheet. We see this for 5v parts too a lot like "6.5v absolute maximum" but we dont want to run it at 6.5v as a normal operating point. 5.5v maybe, but not 6.5v because that is the point where probably many parts failed.
Now we also know that when this kind of test is done that the other specs are held as a max too, so there is also a chance that the unit would survive for a long time if that was the only spec that was violated. So 12v at 25 degree C might work forever, while 12v at 70 degree C may see failures of 50 out of 100 (far too great).
So there's a chance that it could work if no other specs are violated, but it's a risk other wise and it's still a little bit of a risk anyway even if there are no other spec out of whack.
So stick with 10v and you should see long product life. If that's not possible, then modify the circuit a little. Adding two diode in series with the output resistor subtracts about 1.4v from the supply, bringing the max down to 10.6v, and one more diode (three in total) brings it down to about 9.9v which is under the max.