How Do We Identify If an Android Device Is Running Official Android 7.0+ Freeform Multi-Window Mode?

SystemUI uses the following to determine if freeform multi-window mode is available:

mHasFreeformWorkspaceSupport =
    mPm.hasSystemFeature(PackageManager.FEATURE_FREEFORM_WINDOW_MANAGEMENT) ||
            Settings.Global.getInt(context.getContentResolver(),
                    DEVELOPMENT_ENABLE_FREEFORM_WINDOWS_SUPPORT, 0) != 0;

See: https://github.com/android/platform_frameworks_base/blob/nougat-release/packages/SystemUI/src/com/android/systemui/recents/misc/SystemServicesProxy.java#L213-216

DEVELOPMENT_ENABLE_FREEFORM_WINDOWS_SUPPORT is hidden, so you would need to replace that with "enable_freeform_support":

boolean hasFreeFormWorkspaceSupport =
    getPackageManager().hasSystemFeature(PackageManager.FEATURE_FREEFORM_WINDOW_MANAGEMENT) ||
        Settings.Global.getInt(getContentResolver(), "enable_freeform_support", 0) != 0;

I don't have Android-on-Chrome-OS or DeX to test this on, however, this is how SystemUI is checking if freeform multi-window support is enabled. Perhaps you could decompile the settings or systemui app from DeX or Chrome and check if they have made changes here.