The fact that exclude takes dotted notation for nested fields, but unknown does not apply to nested fields, means that exclude doesn't work for nested fields in some common use cases unless the nested schema(s) have the desired Meta.unknown setting, and then they are not runtime-flexible, which is the great benefit of Schema and load taking exclude. Somewhat new to marshmallow so maybe there's a way to accomplish this (setting exclude on nested fields at load time) in a different way?

Happy to contribute a PR if you guys think this is a good idea.