Allow iframe and other tags in WordPress excerpt or content limit

iframe.php

<?php

// Don't Strip HTML from Excerpts or Content Limit
add_filter('get_the_content_limit_allowedtags', 'childtheme_custom_allowedtags');
function childtheme_custom_allowedtags() {
	return '<script>,<style>,<strong>,<b>,<br>,<em>,<iframe>'; //add whatever tags you want to this string
}

?>