You guys forgot the most important and fantastic update in MY 2018: a new windshield wiper fluid level sensor!
Other than that and the changes you guys listed, MY 2016 is the only one that allows playing video on the infotainment display and the A8 transmission is unlocked, so tuning is cheaper.
MY 2017 came with the optional F1FTY (50th anniversary) special edition.
MY 2019 introduced a brand new infotainment software platform with higher resolution screens (1080p instead of 720p), plus the upgraded wireless charging pad (works with newer phones now). It also introduced the optional electronic "rear vision camera" mirror that replaces the traditional rear view mirror and eliminates most rear visibility issues.
MY 2020 is nearly the same as 2019, the new LT1 trim was introduced, plus the rear vision camera was moved into the shark fin antenna from the middle stanchion of the standard SS/ZL1 spoiler.
MY 2021 and 2022 are pretty much the same as MY 2020, except the GM performance air intake is no longer offered.
A couple of color changes, styling packages, slight wheel design alterations, a few newly configurable options such as red seat belts on top of these, but that's pretty much it. The powertrains are exactly the same, except for the lower spec A10 transmission progressively replacing the A8 in 2019-2020 for several trims.
This is not an exhaustive or authoritative list, just the things I remembered. We have a few dealers in the forums, perhaps they can chime in with actual detailed info from the horse's mouth.