Minimal shortcode to override WordPress virtual ban of iframes

iframe_shortcode.php

<?php
function iframe_render_shortcode($atts) {
    $keys = array_flip(array_keys($atts));
    $params = shortcode_atts($keys, $atts);
    $out = [];
    foreach($params as $key => $val) {
        $out[] = $key . '="' . $val . '"';
    }
    return '<iframe ' . implode(' ', $out) . '></iframe>';
}
add_shortcode('iframe', 'iframe_render_shortcode');