proposed class for adding different taxonomies into the search tag interface on search iq

class-SearchIQ.php

<?php

/**
 * If this file is called directly, abort.
 */
if ( !defined( 'ABSPATH' ) ) {
    die( 'Direct access is forbidden.' );
  }

class SearchIQ{

    function __construct(){
        add_filter( 'searchiq_add_tags', array( $this, 'searchiq_add_tags'), 10, 2 );
        add_action( 'init', array($this, 'add_search_tag_taxonomy'), 0 );
    }


    public function searchiq_add_tags( $tags, $post ){
        $terms = get_the_terms( $post, 'search_tag');
        if( !empty( $terms ) ){
            foreach( $terms as $term ){
                $tags[] = $term->name;
            }
        }
        return $tags;
    }

    function add_search_tag_taxonomy() {

        $labels = array(
            'name'                       => _x( 'Search Tags', 'Taxonomy General Name', 'text_domain' ),
            'singular_name'              => _x( 'Search Tag', 'Taxonomy Singular Name', 'text_domain' ),
            'menu_name'                  => __( 'Search Tag', 'text_domain' ),
            'all_items'                  => __( 'All Search Tags', 'text_domain' ),
            'parent_item'                => __( 'Parent Search Tag', 'text_domain' ),
            'parent_item_colon'          => __( 'Parent Search Tag:', 'text_domain' ),
            'new_item_name'              => __( 'New Search Tag Name', 'text_domain' ),
            'add_new_item'               => __( 'Add New Search Tag', 'text_domain' ),
            'edit_item'                  => __( 'Edit Search Tag', 'text_domain' ),
            'update_item'                => __( 'Update Search Tag', 'text_domain' ),
            'view_item'                  => __( 'View Search Tag', 'text_domain' ),
            'separate_items_with_commas' => __( 'Separate search tags with commas', 'text_domain' ),
            'add_or_remove_items'        => __( 'Add or remove search tags', 'text_domain' ),
            'choose_from_most_used'      => __( 'Choose from the most used', 'text_domain' ),
            'popular_items'              => __( 'Popular Search Tags', 'text_domain' ),
            'search_items'               => __( 'Search Search Tags', 'text_domain' ),
            'not_found'                  => __( 'Not Found', 'text_domain' ),
            'no_terms'                   => __( 'No search tags', 'text_domain' ),
            'items_list'                 => __( 'Search Tags list', 'text_domain' ),
            'items_list_navigation'      => __( 'Search Tags list navigation', 'text_domain' ),
        );
        $args = array(
            'labels'                     => $labels,
            'hierarchical'               => false,
            'public'                     => true,
            'show_ui'                    => true,
            'show_admin_column'          => true,
            'show_in_nav_menus'          => false,
            'show_tagcloud'              => true,
            'rewrite'                    => false,
            'show_in_rest'               => true,
        );
        register_taxonomy( 'search_tag', array( 'post', 'page' ), $args );
    
    }
}

add

$tags = apply_filters( 'searchiq_add_tags', $tags, $post);

to library/core.php on line 1021