Hi Henry,
The API gets the data from the live registry so it should reflect the situation at the ICP at the time you query it. To be very clear its the latest available state of information.
The API was released primarily to assist third parties who do not have access to the registry get the relevant information they need to offer informed advice to consumers.
The EMI API is not an enhancement to core registry functionality for participants. The EMI API essentially piggybacks on the web services already available to participants. That said, it is available to anyone, including participants. Calling the API with all ICPs (if you had them) all the time was not the intended usage and we'd need to manage the load this API placed on the registry.
It's still unclear why you want this information and how you would find it useful? For example, what purpose would be served by finding out the metering configuration three years earlier at an ICP? One might expect metering configuration data to change fairly slowly.
Cheers,
Matthew