The Java driver should support BsonExtraElements annotation on POJOs. This flag would provide the ability to leave any extra document elements untouched when performing encoding and decoding.

This feature can be helpful in when performing schema migrations, as it may reduce the need update applications to handle different versions of schemas.

This flag already exists in other drivers such as C#: https://mongodb.github.io/mongo-csharp-driver/2.14/apidocs/html/T_MongoDB_Bson_Serialization_Attributes_BsonExtraElementsAttribute.htm