The API's version is in its path: /api/v1. The API reference shows the version of the document it renders.
Additive changes — a new endpoint, a new optional field, a new event type or enum value — are made within v1. Build your client to ignore fields and event types it does not know.
Breaking changes come as /api/v2, served beside /api/v1 for at least 12 months.
An endpoint that is being retired answers with a Deprecation header and a Sunset header giving the date after which it may be removed. Watch for both in your logs.
Changes are listed in the changelog published with each release.