WordPress: how to set a default category for a custom post type

set-default-category-for-custom-post-type.php

<?php

add_action('save_post', function($post_id, $post) {
    $custom_post_type = 'custom-post-type';
    $custom_taxonomy = 'custom-taxonomy';
    $default_term_slug = 'default-term-slug';

    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return;

    // If this is just a revision, don't set default category
    if (wp_is_post_revision($post_ID)) return;

    if ($post->post_type !== $custom_post_type) return;

    if (!in_array($post->post_status, ['publish', 'draft'])) return;

    // Only set default category if no terms are set yet
    $terms = wp_get_post_terms($post_id, $custom_taxonomy);
    if (!empty($terms)) return;

    $default_term = get_term_by('slug', $default_term_slug, $custom_taxonomy);
    if (empty($default_term))  return;

    // Assign the default category
    wp_set_object_terms($post_id, $default_term->term_id, $custom_taxonomy);
}, 10, 2);