Add's a hash (#) to the front of a tag on the frontend of WordPress

post-tag-add-hash-prefix.php

<?php
add_filter('get_term', function ($term, $taxonomy) {
  // Bail if we're in the admin
  if( is_admin() ) {
      return $term;
  }

  $already_modified = strpos( $term->name, '#' ) === 0;

  if ( $taxonomy === 'post_tag' && ! $already_modified ) {
    $term->name = '#' . $term->name;
  }

  return $term;
}, 10, 2);