There doesn't seem to be a standard.
The StackOverflow answer leans towards 410 GONE, but I think 301 MOVED PERMANENTLY is more appropriate.
To make the correct choice, we have to look at your specific case. If your goal is to have all calls being made to API v1 fail without taking any further action, 410 GONE works for that. If you want some continuity, such as redirecting the client to a newer version of your API where their call may succeed, 3XX works, but which do you choose? I think that if you're trying to shut down API v1, 301 MOVED PERMANENTLY helps indicate that better than 303 SEE OTHER because 301 suggests that all future requests should be made to the new url whereas 303 does not indicate whether or not this situation is permanent.
I would recommend engineering the API in such a way that each version remains backwards compatible, so that 301 MOVED PERMANENTLY would transparently keep your API alive and up to date whenever you add new endpoints for new API versions. I think that's what you're trying to do anyway.
HTTP Status Codes
HTTP status code 302 was originally too broad and thus became incorrectly implemented/used, so 303 and 307 were made to distinguish between 302's dual use case. Some APIs use 303 for other purposes.
301 MOVED PERMANENTLY - The 301 (Moved Permanently) status code indicates that the target resource has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs.
302 FOUND - The 302 (Found) status code indicates that the target resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client ought to continue to use the effective request URI for future requests.
303 SEE OTHER - A 303 response to a GET request indicates that the origin server does not have a representation of the target resource that can be transferred by the server over HTTP. However, the Location field value refers to a resource that is descriptive of the target resource, such that making a retrieval request on that other resource might result in a representation that is useful to recipients without implying that it represents the original target resource.
410 GONE - The 410 (Gone) status code indicates that access to the target resource is no longer available at the origin server and that this condition is likely to be permanent. If the origin server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (Not Found) ought to be used instead.
How do existing APIs handle this?
Maybe you can take a page from Google's Youtube API:
When an API request fails, YouTube will return an HTTP 4xx or 5xx
response code that generically identifies the failure as well as an
XML response that provides more specific information about the
error(s) that caused the failure. For each error, the XML response
includes a domain element, code element and possibly a location