Just found out that [OptionalFieldAttribute] does not work on Mono, and therefore on Unity 😦
Actually a new field in a class should not throw any error messages on older applications using older versions of this class.
But unfortunately it does throw an error on Mono and there is no way of preventing that on the older applications 😦
Here some links about it:
and if you are still curios about it, BinaryFormatter.AssemblyFormat