I think it would be useful to standardize the approach to versioning the API. For example, when I want to change the request or response format for some API endpoint, which is backwards incompatible. How should the client tell which API version to use?

There are many different approaches to this: put version in url like /api/1/action or /api/v1/action or in the header, like "Content-Type: application/vnd.company+json.v1" or "X-API-Version: 1".

How about explicitly including the strategy to versioning API in this specification?