Unsupported Windows is a Windows version that no longer receives standard security servicing from Microsoft. That usually means no regular security patches, feature updates, or servicing changes that keep the update chain trusted and functional.
This matters because endpoint security depends on more than malware detection: it depends on timely fixes and on the ability to verify signed updates from Microsoft. Attackers often target unsupported systems because known vulnerabilities can remain unpatched for years. Defenders treat unsupported Windows as a high-risk asset, isolating it, restricting network access, or migrating it to a supported release. In practice, support status is part of patch management and trust management: if the platform is out of support, the system may still run, but it is outside the normal security maintenance path.