I cannot see how a passenger subsequently dying of Covid could ever be an issue for an airline in legal terms.
Imagine somebody travels to an airport, goes through customs, sits in waiting areas, uses the restroom, gets on a plane, collects baggage, does customs again, then travels by car/bus/train to their home, or a hotel etc etc...and on it goes...
A week later they succumb to Covid. I would defy anyone to prove in a court that they contracted it on the flight. Maybe they already had it before they travelled. Maybe over the course of a week, they caught it at some time other than the few hours they were in a plane, wearing a mask?
I think the airlines are fireproof on this one ( but I am NOT a lawyer).