As T.j said, worst kept secret. It was being discussed on these forums 3 years ago!
https://forums.frontier.co.uk/threads/planet-safari-next-up.482022/ (you will also notice one of the names on their list is Planet Zoo, not forgetting they made Zoo Tycoon.)
Planet Coaster was released, then JWE which obviously was used for animals, a Safari or Zoo DLC was dismissed as coming for Planet Coaster. Add them all together and the next logical step was this being announced for the game being released this year. Plus of course the amount of time that people in the industry would have known about this for means it's much easier to get about.
The "leak" is still as transparent as it was the day it surfaced.