Not necessarily, people who would like extinct animals in their zoo could still have them alongside still existing animals if they're part of dlc, and I think there's a bunch of species that would make sense more than other. Basically the more recent the extinction, the less problematic it would be. The problem is that most people would not have the same definition of how far you can go before it's too much.Then there have to be a separate game featuring zoos for extinct ones like dinosaurs, mammoths and dodos. There's already Prehistoric Kingdom being like that but of different developer. Jurassic World Evolution was Frontier Development game too but creatures there had inaccurate body parts so the other zoo game will/could have accurate ways.
I'd say the possible lines would be:
- species has been studied by modern zoologists, we have photographic or video recordings (thycaline, Bali/Java/Caspian tigers, passenger pigeon, quagga...)
- species has been described/drawn in details by naturalists, we have stuffed animals (Falkland islands wolf, dodo, great auk...)
- species has been described by Ancient or Medieval authors whose writings have survived to this day, can be identified on Ancient or Medieval graphic representations, or is still part of oral stories in which they can be clearly identified (South Island giant moa, Haast's eagle, aurochs...)
- species has been confirmed to have interacted with humans before written records (cave lion, cave bear, mammoth, whooly rhinoceros, Irish elk, megatherium...)
- species lived during the Cenozoic era (smilodon, paraceratherium, dire wolf, gastornis, hell pig...)
- species lived earlier than the Cenozoic era (any non avian dinosaur, pterosaurs, carboniferous giant arthropods.......)
Personally, the extreme limit for me is somewhere around 3 and everything beyond that should be part of an entirely different game.