I have the same issue, I'm not able to reach the same side bar appearance as the Settings app with UISplitViewController (tested on iOS 26 Beta 4). I've created a very basic double column split view controller and there isn't the Liquid Glass effect around the edges of the primary view controller. Interestingly, it does work if one uses the SwiftUI equivalent NavigationSplitView. So either it's a bug in UISplitViewController, or it's not planned for it to support that appearance (hopefully not!).
Topic:
UI Frameworks
SubTopic:
UIKit
Tags: