I open this post to bring attention to overlooked UIManagedDocument. It still marked with @MainActor despite the fact that UIDocument is gonna be nonisolated in iOS 26.1.
Here is what we will have with iOS 26.1 (the fix was there since, at least, beta 2):
UIDocument was incorrectly annotated to be main actor only. This has been corrected. As a result you might see new warnings or errors show up in your Swift code using UIDocument. (149990945)
Seems that UIManagedDocument should also have to be corrected the same way. Otherwise, in Swift 6 we will have crashes.
Feedback report for this issue is FB20555456.