Since .NET and Xamarin were open sourced, I’ve kept a casual watch on their repositories for interesting things.
Recently I noticed this pull request “Change appearance of NSTabView”. This immediately stood out since NSTabView is an AppKit (i.e. macOS) API as opposed to a UIKit (i.e iOS) API.
A quick read of the details and is indeed about some difference between NSTabView and UITabBar. The pull request is targeting the branch “macos” and it seems it’s been in development since at least August 17th.