WhatsApp has taken the dark mode to Android already, and that has been there for some weeks now. While we were expecting that they will put Apple device users on the same train soon, though, they are stalling more than necessary. However, it is sure that the company behind this app is working hard on adding a Dark Mode for iOS users.
This is evident in how the beta testing version of WhatsApp for iOS has detailed a testing of the dark mode on the iOS. If this is at hand, then it means that we are a step closer to getting the update. As of the time of this writing, we can predict that the dark mode should come with the next version of WhatsApp to show up on the official App Store.
We seriously hope the wait is worth it at the end of the day since app developers prioritizes for iOS over Android