Some people actually explained the issue with heterochromia here: https://forums.ea.com/discussions/the-sims-4-feedback-en/sims-4-oversights-in-2025/11262803
1) humans (...) were not designed to have it and thus the game doesn't "support" it, while animals were designed to have it and the game does.
2) There's only space on the texture sheet for a single eye, and the texture gets copied to both of a sim's eyes.
This means the eyes colors can't be different from each other since they repeat itself in both sides so that's a no for complete heterochromia, but a maybe for sectorial heterochromia, if we ever accept having both eyes having the same sectorial detail.