The created_at field in v5 was of no real value to anyone. It had no relation to when a user actually subscribed but rather when that subscription object was created. Because of this it meant anything like a change in payment method, or potentially other things, would create a new subscription object with a new created_at despite no change to the users subscription status.
As for additional fields such as months in advance a user subbed for, you should submit feature requests on https://twitch.uservoice.com/forums/310213-developers (or upvote an existing request if one exists for that).