Microsoft added a known issue to the October Windows 11 security update KB5127648: on some 24H2-and-later devices where Smart App Control was turned off, the toggle in Windows Security is grayed out and can't be turned back on. Devices with it already on stay protected. There's no fix yet. Microsoft's workarounds are two reg add FeatureManagement override values under HKLM\SYSTEM\CurrentControlSet\Policies\Microsoft\FeatureManagement\Overrides (listed in the KB) followed by a restart, or, for managed fleets, installing and enabling both 'KB5074105 261015_19222/19221 Feature Preview' Group Policies. Don't skip the security update over this; if SAC matters to your baseline, check which devices report it off before deploying.