Hello,
You can still use the Remote Desktop Client. Although Microsoft is transitioning to the new Windows App, the traditional Remote Desktop Client remains available and fully functional. This is likely because the new Windows app is still in preview and many features are yet to be refined.
However, Microsoft is no longer updating the Remote Desktop client in the Microsoft Store with new features for Windows.
For the macOS client, it is clearly indicated that the Remote Desktop app is transitioning to the new Windows App.
For the best experience, especially if you use Azure Virtual Desktop, Microsoft recommends switching to the new Windows App.
Regarding tutorials for this app, the only resource available is the official documentation:
Windows App user documentation - Windows App | Microsoft Learn
Be sure to review the Known issues and limitations of Windows App
You can always provide feedback through the Feedback tab within the app.
I hope this information is helpful.
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