What is the authoritative source(s) for the ATmega32U4 (or any Arduino chip for that matter) datasheet?
4/2016 does appear to be the latest datasheet for the Atmega32U4. If you go here www.microchip.com and select Design, then Documentation, then Datasheets, then Search by Products, you can enter a part number. Entering Atmega328P brings up the complete datasheet dated 1 Nov 2018. You can check that to see if there is more characterization data.
(question was originally about characerization of ADC for '32U4 and other sources of info on same, hence the '328P reference here).