Hi Matteo Pagani ,
one question regarding your caveat at the end of your article: "You can't publish a modification package or an optional package on the Microsoft Store."
Why the heck this restriction? It is obviously not a technical thing, since optional packages and related API are already around for a while in UWP. Documation mentions also nice things like separate licensing for those optional packages (which would be great, shipping optional packages only when users really need them and bought an associated in-app purchase!)
"To submit an app that uses optional packages and/or related sets to the Microsoft Store, you will need permission. "
[https://docs.microsoft.com/en-us/windows/msix/package/optional-packages]
Once again, this is a store feature that is available - but not for everyone.