Food that has been frozen for too long is theoretically edible if it has not gone bad. The reason why food is inedible is that the nature of food has changed and some substances harmful to human body have been produced. For example, spoiled food is likely to produce toxins harmful to human body, and people will be poisoned if they eat such food. Besides, the chickens and ducks you mentioned have been frozen for a long time (half a year). According to our existing science, animal carcasses at minus 18 degrees will not deteriorate for half a year. In other words, you should be safe to eat, and you won't have adverse effects because of eating these chickens and ducks. But food frozen for a long time is not as delicious as fresh food, and the taste will definitely get worse.
From a biological point of view, due to long-term death and long-term freezing, all the cells in the chicken body are necrotic, and various nutrients may have degraded due to abnormal metabolism! However, due to long-term freezing, it caused a lot of dehydration and accumulated more protein! Because it should be edible, but its nutritional value is far from fresh …