How to truncate menu label in wordpress?

Looks like the filter nav_menu_item_title is what you’re after. Here’s an example to use that to truncate the title to 10 characters. This is untested, but should do what you want, added to e.g. your functions.php

    function filter_nav_menu_item_title( $title, $item, $args, $depth ) { 
        $truncLength = 10;
        if (strlen($title) > $truncLength) {
            return substr($title, 0, $truncLength) . "...";
        } else {
            return $title;
        } 
    }
         
    // add the filter 
    add_filter( 'nav_menu_item_title', 'filter_nav_menu_item_title', 10, 4 );

Does that do it?