Wordpress - Show all terms of a custom taxonomy?

You need to pass an additional argument to get_terms(). The default is to hide "empty" terms-- terms which are assigned to no posts.

$terms = get_terms([
    'taxonomy' => $taxonomy,
    'hide_empty' => false,
]);

Since 4.5.0, taxonomies should be passed via the ‘taxonomy’ argument in the $args array so:

$terms = get_terms( array(
    'taxonomy' => 'post_tag',
    'hide_empty' => false,
) );

where terms that have no posts are hidden by default.

This code is fetches all category and subcategory custom taxonomies using get_terms():

<?php $wcatTerms = get_terms('product_cat', array('hide_empty' => 0, 'parent' =>0)); 
   foreach($wcatTerms as $wcatTerm) : 
   ?>
<ul>
   <li>
      <a href="<?php echo get_term_link( $wcatTerm->slug, $wcatTerm->taxonomy ); ?>"><?php echo $wcatTerm->name; ?></a>
      <ul class="megaSubCat">
         <?php
            $wsubargs = array(
               'hierarchical' => 1,
               'show_option_none' => '',
               'hide_empty' => 0,
               'parent' => $wcatTerm->term_id,
               'taxonomy' => 'product_cat'
            );
            $wsubcats = get_categories($wsubargs);
            foreach ($wsubcats as $wsc):
            ?>
         <li><a href="<?php echo get_term_link( $wsc->slug, $wsc->taxonomy );?>"><?php echo $wsc->name;?></a></li>
         <?php
            endforeach;
            ?>  
      </ul>
   </li>
</ul>
<?php 
   endforeach; 
   ?>