Use a custom taxonomy term to hide posts from main queries

hide-taxonomy-term.php

<?php

// Hide custom taxonomy term from all queries
// Still accessible via direct link

add_action('pre_get_posts', function($query){
  if ( $query->is_home() || $query->is_feed() ||  $query->is_search() || $query->is_archive() ) {
    $tax_query = array(
      array(
          'taxonomy' => 'admin_category', // taxonomy name
          'field' => 'slug',
          'terms' => 'hidden', // taxonomy slug
          'operator' => 'NOT IN',
      )
    );
    $query->set( 'tax_query', $tax_query );
  }
});