Figured it out thanks to some help from another engineer: You now have to select "Apply Once to Folder" as the value for Build Rules in the file inspector.

My hunch is that the UNUserNotificationCenter code hasn't gotten the Swift 6/strict concurrency checking treatment by Apple yet. I encountered a similar problem with QLPreviewControllerDelegate and learned of a syntax where you can prepend @preconcurrency to the protocol conformance: final class NotificationServiceDelegate: UIResponder, UIApplicationDelegate, @preconcurrency UNUserNotificationCenterDelegate ... This will let you remove the nonisolated from the protocol methods and fix (or at least silence) the error. A side note for those reading, you'll want to use the async versions of User Notification methods in Swift 6 code where available. The completion-handler-based methods can crash with a __dispatch_queue_assert assertion.

IMO this definitely should be marked @MainActor (reported as FB14256770) but thanks for the workaround!