This is because the OAuthSwift project doesn't have the "Allow app extension API only" checkbox checked. I could submit a pull request to OAuthSwift to make the change, but who knows if it will get accepted.

Let’s make the change in our own fork: https://github.com/Ranchero-Software/OAuthSwift

(Let’s also do a PR to the main repo, to be good citizens.)

This is no longer an issue now that we are using SPM to manage this dependency.