This isn't true. At a wildlife park that I worked at we regularly fed out surplus wild boar piglets which had outgrown their 'cuteness' and were humanely killed for feeding out to large carnivores. They were impossible to rehome. We were upfront with the general public and the local authority was well aware of the situation. Never had any issues.This is illegal in the UK. ALL animals dying in a zoo, from whatever cause, are classed as a Category-1or2 Animal By-Product and must be destroyed or sent to an approved scientific institution such as a recognised museum. None can be used for animal feed, even those from spp specifically bred for the purpose elsewhere. So a wild red deer for example, can be fed to zoo animals without restriction as it is classed as 'game'. If it comes from a deer-farm then it is an agricultural product and has to be registered and ear-tagged but can be fed to zoo animals provided the correct paper-trail is kept. If it was from a zoo origin, then it has to be destroyed or sent to a museum. Zoos cannot produce their own food, unless a separate business is set up on a separate property.
Admittedly most of the meat that we fed out came from a local abattoir or farmers (rifle-shot rabbits, pigeons). Even had a council ask if I was interested in legally shot Canada geese from local parks. And I was well on top of animal by-product rules at the time and how waste was dealt with. However if I regularly fed the wild boar any processed food meant for humans, then I could have lost the licence!