Consider using a parameter in the Content-Type header

[e0ipso]

api_version/src/EventSubscriber/ResponseEventSubscriber.php

Line 26 in b28ba4b

$event->getResponse()->headers->set('X-Drupal-ApiVersion', $this->apiVersionCalculator->getApiVersion()->toSemanticString());

If we use a parameter in the content type header: Content-Type: application/vnd.api+json; version=2.9.18 then we don't have to worry about adding X-Drupal-ApiVersion to the Vary header to support cache variations (even then the support of Vary is not great).