For example, with Notepad++ you can right click a file and you see an "Edit with Notepad++" item. Because these context menus items are added via registry (and the MSIX container-izes it), the OS is unaware of the new context menu item.
You can add context menu with MSIX, but you can do this only with File Type Associations (like in VLC for example) that point to an executable (you can call this a shell menu).
Context menus are not added via the registry in MSIX, these must be added in the AppManifest.xml.
@alexmarin89 yes, I absolutely understand what you're saying but it's not addressing the issue. Let me choose a different example - let's say MediaInfo as the example. MediaInfo is a program that can open a video file and give you various details such as codec, bitrate, etc.
In this case, I wouldn't want MediaInfo to be the default program to open a video file (as obviously playback would be used more often). MediaInfo's installer adds a right-click menu option but this is not supported by MSIX. While your file association workaround would absolutely work for some programs, programs like MediaInfo need right-click menu support for best usability.
If your app is under active development, you can add a context menu entry for your app via a File Type Association and verb registration in your AppxManifest.xml (or package.appxmanifest if you're using the Visual Studio packaging project). Have a look at this example for more details - Modernize existing desktop apps using Desktop Bridge | Microsoft Docs
