Override IriConverter and allow an ApiResource to not have any item route associated with it.

api_resources.yml

resources:
    AppBundle\Api\DTO\SomeDTOWithNoItemOperations:
        collectionOperations:
            get:
                method: 'GET'
        itemOperations: []
        attributes:
            normalization_context: {'groups': []}
            iri: 'referencing_with_conditions'

IriConverter.php

<?php

/*
 * This file is part of the API Platform project.
 *
 * (c) Kévin Dunglas <dunglas@gmail.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

declare(strict_types=1);

namespace AppBundle\Api\Routing;

use ApiPlatform\Core\Api\IriConverterInterface;
use ApiPlatform\Core\Api\UrlGeneratorInterface;
use ApiPlatform\Core\Metadata\Resource\Factory\ResourceMetadataFactoryInterface;
use ApiPlatform\Core\Util\ClassInfoTrait;

/**
 * {@inheritdoc}
 *
 * @author Kévin Dunglas <dunglas@gmail.com>
 */
final class IriConverter implements IriConverterInterface
{
    use ClassInfoTrait;

    public function __construct(ResourceMetadataFactoryInterface $resourceMetadataFactory, IriConverterInterface $decorated)
    {
        $this->resourceMetadataFactory = $resourceMetadataFactory;
        $this->decorated = $decorated;
    }

    /**
     * {@inheritdoc}
     */
    public function getItemFromIri(string $iri, array $context = [])
    {
        return $this->decorated->getItemFromIri($iri, $context);
    }

    /**
     * {@inheritdoc}
     */
    public function getIriFromItem($item, int $referenceType = UrlGeneratorInterface::ABS_PATH): string
    {
        $resourceClass = $this->getObjectClass($item);
        $metadata = $this->resourceMetadataFactory->create($resourceClass);
        if ($metadata->getAttribute('iri')) {
            return $metadata->getAttribute('iri');
        }

        return $this->decorated->getIriFromItem($item, $referenceType);
    }

    /**
     * {@inheritdoc}
     */
    public function getIriFromResourceClass(string $resourceClass, int $referenceType = UrlGeneratorInterface::ABS_PATH): string
    {
        return $this->decorated->getIriFromResourceClass($resourceClass, $referenceType);
    }

    /**
     * {@inheritdoc}
     */
    public function getItemIriFromResourceClass(string $resourceClass, array $identifiers, int $referenceType = UrlGeneratorInterface::ABS_PATH): string
    {
        return $this->decorated->getItemIriFromResourceClass($resourceClass, $identifiers, $referenceType);
    }

    /**
     * {@inheritdoc}
     */
    public function getSubresourceIriFromResourceClass(string $resourceClass, array $context, int $referenceType = UrlGeneratorInterface::ABS_PATH): string
    {
        return $this->decorated->getSubresourceIriFromResourceClass($resourceClass, $context, $referenceType);
    }
}

services.yml

services:
    AppBundle\Api\Routing\IriConverter:
      decorates: 'api_platform.iri_converter'
      arguments:
        $resourceMetadataFactory:  '@api_platform.metadata.resource.metadata_factory'
        $decorated: '@AppBundle\Api\Routing\IriConverter.inner'