WordPress: Include Taxonomies in Editor Link Selection Modal at the backend

functions.php

<?php

/**
 * Filters the link query results. (Link Edit Modal in the editor)
 * @see https://github.com/WordPress/WordPress/blob/3ba44120d0ffa7ac330a1b7e3f8b363ce78f8060/wp-includes/class-wp-editor.php#L1767
 */
add_filter(
    'wp_link_query', function( array $results, array $query ) : array {
        $terms = new WP_Term_Query(
            [
                'taxonomy' => ['product_category', 'room', 'brand', 'category'],
                'search' => $query['s'],
                'relevanssi' => true, // Only if using relevanssi (improves search accuracy drastically)
                'hide_empty' => false,
                'update_term_meta_cache' => false,
                'number' => 20, // Note: You cannot change this, as the original post query hardcodes 20
                'offset' => $query['offset']
            ]
        );

        /**
         * @var WP_Term
         */
        foreach ($terms->terms as $term) {
            $results[] = [
                'ID' => 'term_' . $term->term_id,
                'title' => trim(esc_html(strip_tags($term->name))),
                'permalink' => get_term_link($term),
                'info' => $term->taxonomy
            ];
        }

        usort(
            $results, function($a, $b) {
                return strnatcasecmp($a['title'], $b['title']);
            }
        );

        return $results;
    }, 10, 2
);