Get related tags for category archives

get_related_tags.php

<?php
function get_related_tags() {
    if ( ! is_category() ) {
        // abort if not on a category archive
        return;
    }
    $current_term = get_queried_object()->term_id;
    $related_tags = array();
    $html = ''; 
    
    $related = new WP_Query( array(
        'post_type'     => 'post',
        'posts_per_page'=> -1,
        'fields'        => 'ids',
        'category__in'  => $current_term,
    ) );
    
    if( $related->have_posts() ) {
        foreach( $related->posts as $post_id ) {
            $tags = wp_get_post_tags( $post_id );
            if( ! empty( $tags ) ) {
                foreach( $tags as $tag ) {
                    if( empty( $related_tags ) || ! array_key_exists( $tag->id, $related_tags ) ) {
                        $related_tags[ $tag->term_id ] = $tag->name;
                    }
                }
            }
        }
        wp_reset_postdata();
    }
    if( ! empty( $related_tags ) ) {
        $html = '<ul>';
        foreach( $related_tags as $tag_id => $tag_name ) {
            $html .= sprintf('<li><a href="%1$s">%2$s</a></li>',
                get_tag_link( $tag_id ),
                $tag_name
            );
        }
        $html .= '</ul>';
        return $html;
    }
}