Whether an attribute is visible on a derived class by default is determined by the AttributeUsageAttribute.Inherited property. Which is false for this attribute:
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Constructor,
AllowMultiple = false, Inherited = false)]
public sealed class TargetedPatchingOptOutAttribute : Attribute {
// etc...
}
It is not exclusive, a method like Type.GetCustomAttribute() also has an inherit argument to allow overriding the default.
Do be careful about making assumptions about what tooling reads an attribute. [TargetedPatchingOptOut] is almost certainly not read by the jitter. It is used to determine if Ngen.exe needs to run to re-create the native image of an assembly when an update is deployed. I don't know what the tooling looks like, the only thing that makes sense to me is that this is done on Microsoft machine in Redmond.