That's really a non-issue though. Just because they can decide to receive less information, it doesn't follow that they should be allowed to receive more. I don't see why this particular type of symmetry would be important or desirable.
The way I've implemented this in the past is to start with a standard API endpoint with a defined data-set that it returns. Then I allow the client to select to receive only a subset of those fields, or make no selection and receive all of the defined fields. The client cannot request fields that are not part of the data-set defined by that endpoint. This is at least as easy to work with going forward as an inflexible return object. The API can be updated to include more data without affecting clients who don't care about that. If there is a breaking change, then that requires a new version, just as it would normally.
The way I've implemented this in the past is to start with a standard API endpoint with a defined data-set that it returns. Then I allow the client to select to receive only a subset of those fields, or make no selection and receive all of the defined fields. The client cannot request fields that are not part of the data-set defined by that endpoint. This is at least as easy to work with going forward as an inflexible return object. The API can be updated to include more data without affecting clients who don't care about that. If there is a breaking change, then that requires a new version, just as it would normally.