Hey @Yishar,
The package is still supported but just internally. That just means feedbacks from Unity employees are prioritized for now to improve the package (since it is used by some teams internally). We cannot give the guarantee to end users to solve all the issues they found in the package due to the small amount of persons working on this project. But as you can see the package still release bug fixes here and there.
App UI components support UITK runtime bindings. It is really up to the user if they want to adopt others concepts provided in the package, such as ObservableObject for MVVM patterns, the Context API for propagating data into your visual tree, or Redux implementation for app state management. They are completely optional and should be conflict-free with any UITK initiative.
App UI package has been created for internal 3D apps development at the beginning. Now it is used more widely for different cases. If some features look similar to what is in the UITK roadmap, it is just because we needed them urgently, without the “feature complete” stamp. But still they usually cover 90% of the cases.