I had the opportunity to use Drupal as a headless CMS and tried the Disable UI module, which restricts UI access to administrators and similar roles.
https://www.drupal.org/project/disable_ui
As a result, access to the top page was displayed as follows.

On the other hand, the enabled JSON:API was accessible to non-logged-in users.
/jsonapi/

We hope this serves as a useful reference when using Drupal as a headless CMS.