With Square’s API life cycle deprecated is the first stage toward retirement. They are typically deprecated at least 12 months before permanent retirement.
Deprecated functionality remains publicly available and fully supported, but its use is strongly discouraged for all applications, regardless of the Square-Version provided with API calls. You are discouraged from building new solutions with deprecated APIs. On deprecation, you should plan on updating existing applications that depend on the deprecated APIs.
The Auth header version overrides the default by explicitly specifying an API version in a request by adding the Square-Version request headers. This enables developers to test different API versions. 