Not available in general. It's a long story, but the original iOS app was pretty alpha, had an obsolete codebase, and wasn't working well. We unpublished it from the app store a few weeks ago, since it was worse than just using the website on mobile.

The development community has been rewriting the iOS app in React Native over the last couple months, and the new version is working really well! It'll be in the app store in a few weeks. See https://github.com/zulip/zulip-mobile.