Grizzly x Polar bear hybrids are still not "natural". The only reason it's happening now is due to the fact polar bears are having to move further and further south because of loss of habitat in their normal range. Plus we haven't been able to study them enough to know just how detrimental this mixed breeding might have to their health.
Even if it were "natural", at the end of the day the only way to get them in game would be to force them into the same habitat enclosure and breed them, and that there already makes it unnatural and unethical.
There's already so many animal species that could be brought into the game, most that support its advertisement of being about conservation, that I see zero reason to sully that with hybrids. Just my opinion.