OmniFocus 4 for iPhone and iPad do not feature in-app integration with TextExpander.
The SDK that made in-app integration possible was retired by TextExpander in 2022, and they have recommended developers stop using it. In place of the old SDK, TextExpander is working on a new version of their iOS app with a new TextExpander Keyboard, which should provide much of the in-app functionality of the old SDK. TextExpander has a sign-up page, where you can request to join their beta program to test this new version.
