Why isn't the input impedance mentioned in the datasheet of the AD8597 operational amplifier?
As a bipolar-input amplifier, The input impedance of that part is relatively low, less than 1G ohm, typically as you can see from the change in input bias current with CM voltage. Eg. Fig.15.
Typically, to get the benefit of the front end which is run at relatively high current with super beta transistors, you’d want a source impedance less than 1K so that’s of little concern.
The input bias current is typically only 40nA (still quite high) but can be as high as 340nA over temperature, which, along with the large offset voltage, are probably bigger concerns.
It’s optimal for low impedance AC sources where noise is a primary concern.
If high input resistance and bias current is your main concern, you can get CMOS-input amplifiers with bias currents in the double digit fA range, which is more than 7 orders of magnitude less.