Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

There is a new subtitle added to the UINavigationItem on iOS 26 https://developer.apple.com/documentation/uikit/uinavigationitem/subtitle. This conflicts with the existing fluent subtitle so let's make that obsolete for iOS 26.0 to fix build break with Xcode 26 beta.

Verification

Builds fine on Xcode 26 beta. Subtitle shows up on iOS 26 with the system subtitle.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow