By design the module exposes resources for every entity type in the system. However, if your aplication contains an entity which does not implement ContentEntityTypeInterface or ConfigEntityTypeInterface, the module throws a whitescreen/500 error when installing or rebuilding cache, which upon checking the logs says:
LogicException: Only content and config entity types are supported. in Drupal\jsonapi\ResourceType\ResourceTypeRepository->getAllFieldNames() (line 285 of /var/www/html//modules/contrib/jsonapi/src/ResourceType/ResourceTypeRepository.php).
It might not be the usual use case to support non-content/config entities, but it would be nice to handle them in a way that doesn't break the system, like excluding them from resources and relationships.