The use of [Serializable] attribute should be revisited.
According to investigation in scope of CSHARP-2936 it is currently used for making all MongoDB.Driver custom exceptions (e.g. MongoException and BsonException) serializable to binary, which in turn requires all of their fields to be serializable as well, resulting in the [Serializable] attribute being applied to a large amount of classes which do not need it otherwise.
The reason for making custom exceptions [Serializable] is not entirely clear, but is likely connected to transferring these exceptions between different AppDomains (which should be confirmed).
Dropping the [Serializable] attribute is a breaking change, thus it should be considered for major release (if the reason for applying it is not longer valid), but in return will eliminate the need to support it in the codebase.