Per the documentation found here https://developers.clicksend.com/docs/rest/v3/#view-list-contacts
The endpoint https://rest.clicksend.com/v3/lists/{list_id}/contacts supports a query string parameter 'updated_after'.
ContactApi.listsContactsByListIdGet() should support this parameter. Perhaps as something passed in the existing 'options' argument to that method--to maintain backward compatibility.
My use case involves regularly pulling the contents of the Unsubscribe/"Opt-Out List". I really only care about recent additions. As it stands the current method doesn't provide any way to inject arbitrary query string data. So, there's no easy way to implement a workaround in the interim. So, over long enough time periods, this process will get needlessly expensive.