Recently gi-gtk3 (and gi-gtk4) were put on Hackage, the former is basically a drop in replacement for gi-gtk < 4.
They allow easier packaging with both major versions (eg both should be available soon in Stackage Nightly and already in LTS)
allowing us to support both gtk3 and gtk4 apps.

Hence I suggest it would be good to switch taffybar's gi-gtk dependency over to gi-gtk3.