Microsoft has released a new Xbox app for mobile devices in beta form, with a completely rewritten infrastructure plus the addition of Xbox console streaming as a feature for all users.
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Microsoft has officially announced that PC games on the Microsoft Store can now include mod support on a bring-your-own-mod basis. A few Xbox Game Pass titles already support it, with more on the way.
Microsoft is starting to add support for game mods in the Xbox (Beta) app on Windows 10. You can enable mods for select games, but there's no way to get mods from the store itself.