Using environs I noticed that type-casting methods they don't return the type casted, only Unknown,

If the intention is to cast the environment variable to the correct type it should let your IDE know that it is indeed that type being returned. Environs should leverage marshmallow to return the exact type that is being casted. Type checkers like mypy would benefit from this behavior as well.

One more thing is that type-casting methods are not exactly methods and are being treated as variables although the syntax used is like calling functions. Maybe this should be reviewed in the next version?