On iPadOS, recent iPads without M-series SoCs should still be able to run apps on external monitors full screen or in Split View at arbitrary sizes. Not the full Stage Manager experience but apps are still capable of adjusting to various widths.
Also, it should be possible to make notifications from any given app play an alert sound even if the app doesn’t request the ability to do so.
As a developer, on the other hand, there is still far, far too much on the UI side you can do with UIKit or AppKit that you can’t easily do with SwiftUI.