Get Registered Custom Post Type to get All Custom Taxonomies

Try this (tested) code below. I believe the issue was the passing of the $post_type parameter to get_object_taxonomies (i.e. you passed it in quotes as '$post_type'. If uncomment the two var_dump‘s you’ll see what information is outputted.

<?php
    $postargs = array(
       'public'   => true,
       '_builtin' => false
    );

    $post_types = get_post_types( $postargs, 'names', 'and' ); 

    // Main Loop Start
    foreach ($post_types as $post_type){  
        //var_dump($post_type); 
        $taxonomies = get_object_taxonomies($post_type, 'objects');    
        foreach ($taxonomies as $taxonomy){
                //var_dump($taxonomy);

                // Do Stuff with taxonomy here


            }//foreach taxonomy
    }//foreach post_type
?>

Note: I’ve tested this with custom post types registered by a theme, but not with custom post types registered by plugins, but this should work for those too