hydrogen sulfide is not anywhere in the same category. When you consider failure you have to consider what is the most catastrophic possibility and if that is “this battery silently kills people” then you dont make it.
Cyanide could be released only at high temperatures, e.g. if the battery is opened and burned, not during normal operation, even if overcharging is not prevented, as it should.
The sulfuric acid from the traditional lead-acid car batteries is more dangerous than this.
Very much not an equal comparison.
Cyanide could be released only at high temperatures over 300 Celsius degrees.
During a fire, there are many other things in a car that can release toxic fumes easier than a sealed battery.
It has the same LD50 dose as HCN. It literally _is_ just as bad. It routinely kills people on oil rigs because in lethal concentrations it immediately shuts off your nose.
How often do you hear about people getting poisoned by it from lead-acid batteries?
Fast charging a car/chemical weapon in your garage isn't terribly appealing.
https://en.wikipedia.org/wiki/Hydrogen_cyanide - 107 ppm (human, 10 min)
https://en.wikipedia.org/wiki/Hydrogen_sulfide - 600 ppm (human, 30 min)
https://en.wikipedia.org/wiki/Carbon_monoxide - 4000 ppm (human, 30 min)
These are "LCLo" values from the databoxes on those pages. More easily comparable numbers may be around somewhere.