Having euthanized dogs in dog food is very unlikely because when someone takes there pet in to the vet to be euthanized they usually either cremate the dog or bring the dogs body back home to be buried.
However if a dog shelter euthanizes dogs then they might sell off the dogs body to be made into pet and dog food from the remains.
I've heard of a few dog shelters selling off the dogs bodies and other pets bodies to pet food factories who then grind the dog up to make meat out of them.
There's really nothing wrong with that because once the dog dies they would either be cremated or buried and rot in the ground.
So if other dogs and pets can eat the meat of the dogs and other pets then they really should.
Some people do have a problem if dogs are sent off to be pet or dog food but others do not.
Usually though most dog food brands do not contain any dogs that have been euthanized in the dog food.