The changelog provides a list of dated updates, each of which contains a number of potentially backwards-incompatible changes. There is no explicit versioning in the API at this time; all changes will be either compatible or breaking. (If I attempt any brittle changes to the API that may break current implementations, I'll be sure to add version gates.)
Added support for filtering emails in `/v1/emails/` on `included_tags` and `excluded_tags`.
Took the ability to send specific emails to subscribers out of beta; added `/v1/subscribers/<id>/send-reminder`, which allows you to send reminders to your subscribers to confirm their email address.
Added support for Exports via the `/v1/exports` endpoint.
Added support to set and retrieve metadata on Emails.
Added deletion and update abilities to the Scheduled emails endpoint, giving you much more programmability than the hitherto append-only state of the world.
Added a deletion endpoint to the Images endpoint, allowing you to delete unused images.