Drupal 9 page cache does not respect the Vary header

Issue

Drupal Page Cache does not currently respect the Vary header , please see the following issues on Drupal.org for more information:

• Page Cache must respect Vary, otherwise it might serve mismatched responses
• Introduce "Vary" page cache response policy

Affected Applications/Limitations

Any Drupal application with page cache enabled attempting to use the Vary header for variations in cache.

Resolution

As a current workaround you can disable page cache by adding the following snippet in settings.php:

$settings['cache']['bins']['page'] = 'cache.backend.null';