How to filter the terms of custom taxonomy by author id in admin panel

I don’t have enough time to test the full code, but you can get the idea from my code.

Taxonomies themselves don’t have a direct association with authors. So there is no easy way to do it.

You can add custom meta fields to custom taxonomies also. So you can store the current user ID in a custom meta field.

function save_current_user_id_for_customers($term_id, $tt_id, $taxonomy) {
   
    if ($taxonomy == 'customers') {
        
        $current_user_id = get_current_user_id();

        
        update_term_meta($term_id, 'user_id', $current_user_id);
    }
}

add_action('create_term', 'save_current_user_id_for_customers', 10, 3);

Then you can use a parse_tax_query filter to filter customers.

function filter_customers_taxonomy_query($query) {
    global $pagenow;

    
    if ($pagenow == 'edit-tags.php' && isset($_GET['taxonomy']) && $_GET['taxonomy'] == 'customers') {
        
        $current_user_id = get_current_user_id();

        
        $tax_query = array(
            array(
                'taxonomy' => 'customers',
                'field'    => 'user_id', // Change this to the actual custom field name
                'terms'    => $current_user_id,
                'operator' => 'IN',
            ),
        );

        $query->tax_query->queries = $tax_query;
        $query->tax_query->relation = 'AND';
    }
}

add_action('parse_tax_query', 'filter_customers_taxonomy_query');