Safe to disable the product authentication service on Windows 10?

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Technically yes, but it's not recommended unless you have a specific reason (e.g., testing in a controlled environment).

On Windows 10, the Product Authentication Service (sometimes called Windows License Manager Service or similar) is responsible for managing your Windows activation status. It communicates with Microsoft's servers to verify that your copy of Windows is genuine and properly licensed. It helps ensure that your Windows copy isn't pirated and that your system is properly licensed.