Custom field normalizers should define support for custom_field data types.

Created a MR for the basic update. Will continue testing locally, but initial errors have gone away and json:api results are returning properly now.

Thanks for the patch. The EntityReferenceNormalizer however also supports CustomFieldImage so would either need to revise this to support both types or split that off into its own separate normalizer. Open to further review.

btw, i guess this is an oversight around Drupal 9 compatibility as those errors aren't happening in Drupal 10 because of this function:

/**
   * {@inheritdoc}
   */
  public function getSupportedTypes(?string $format): array {
    return [
      CustomFieldEntityReference::class => TRUE,
      CustomFieldImage::class => TRUE,
    ];
  }

This is only an issue in the 2.x version which I'm no longer supporting due to the extent of api changes between versions. If someone comes across this issue for 2.x... the attached patch should work for you but highly encourage updating your site to latest core and switching to the 3.x module cause its way more feature rich.