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.
Windows 11 Beta Channel gets Taskbar, File Explorer improvements, and ISOs
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As per a job listing, Microsoft might be working on implementing game mod support for Xbox Live titles on the Xbox One and Windows 10. The company has not officially announced such plans.