On devices with dual cameras (7+, 8+, and X), you can tap the "2x" to use the optical zoom.
We don't support pinch-to-zoom on other iPhones. We've decided to omit it for a few reasons.
Digital zoom is just a software effect that throws out pixels. Most photographers prefer to crop the full image later in editing. Accidentally triggering pinch-to-zoom, and then losing pixels, would be very annoying.
We know that some people use digital zoom to help compose shots before editing later. Other people use zoom as a focus tool. In these situations, we have great tools planned that are much better than digital zoom. Stay tuned!