Custom ACF select validation

functions.php

<?php

/*
 * Disallow certain choices based on post_type and/or page template
 */

add_filter('acf/prepare_field/key=field_5c6c913e0ea64', function(array $field) {
  //unset($field['choices']['full-width-image-carousel']);
  //unset($field['choices']['full-width-image']);
  return $field;
});
add_filter('acf/validate_value/key=field_5c6c913e0ea64', function(bool $valid, $value, $field, $input) {
  $post = new Timber\Post($_POST['post_id'] ?? $_POST['ID']);
  if (!in_array($value, ['full-width-image', 'full-width-image-carousel'])) {
    return $valid;
  }

  if ( $post->post_type === 'page' ) { // or whatevs
    $valid = "Full-width image carousels are not allowed on pages using the default General Interior template.";
  }
  return $valid;
}, 10, 4);