To disable caching while testing your JSON:API endpoints you should put this into your settings.php
$settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.null';
but for this to work, you need to define this service null, which should be added as
parameters: http.response.debug_cacheability_headers: true services: cache.backend.null: class: Drupal\Core\Cache\NullBackendFactory
this is usually added as development.services.yml and both of this is added to settings.local.php
$settings['container_yamls'] = DRUPAL_ROOT . '/sites/default/development.services.yml'; $settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.null';
Also don't forget to click SHIFT + refresh in your browser so that browser caching is skipped.