Issue: Feature is enabled or disabled to get in customize ISV/VAR/USR model. Since it was defined as internal class by MS, and which can't be accessible in reference model.
Workaround solution (X++) in ISV/VAR/USR model: Without using MS OOB class.
IdentifierName featureEnableFeature = 'Dynamics.AX.Application.<>';
FeatureManagementState featureManagementState;
boolean isFeatureEnabled = false;
select firstonly featureManagementState
where featureManagementState.Name == featureEnableFeature ;
if (featureManagementState.IsEnabled)
{
isFeatureEnabled = true;
}