> Catullus 16, which might be the oldest thing that we could properly call a diss track
This view is a bit disingenuous. There's been diss tracks as long as mankind has been singing and writing poems. While Catullus 16 is very famous, there's much earlier examples. The greek counterpart to Catullus may be, for example, Hipponax of Ephesus [0], who wrote celebrated "diss tracks" six centuries before him. But still, a lot of ancient greek comedy can be understood as unrestricted verbal abuse to concrete people of the time.