I don't know either.
According to AutoWeek, nothing but an Impala/Caprice of the right years can "officially" be a donk. This said, I'll go with too-big wheels like most folks do (as the 2017 AW article's author himself admits).
This said, how about a specific thread for truly donked C's, so that the present thread can keep focusing on more pleasing wheel choices? In the two photos you just posted, for example, I personally would not call the Imperial "donked" -- it indeed remains rather tasteful, whereas the Chrysler's wheels are IMHO truly disproportionate and "donk" the car.