How to set show admin bar front to true for all users?

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You can use update_user_option() function (see codex)

Your loop looks fine to me, so probably this would work:

// Create the WP_User_Query object
$wp_user_query = new WP_User_Query(array('role' => 'Subscriber'));

// Get the results
$users = $wp_user_query->get_results();

// Check for results 
if (!empty($users)) {

    // loop trough each author
    foreach ($users as $user)
    {
        // update option
        update_user_option( $user->ID, 'show_admin_bar_front', 'true');
    }
}

Things to pay attention to:

  1. You’re looping through all queried users, so in update_user_option
    the first parameter needs to be the id retrieved from current user
    object (not the hardcoded ID)

  2. The third parameter in update_user_option should be of a string type apparently, so "true", not true