wp_kses_post_tags.php

/**
 * Add iFrame to allowed wp_kses_post tags
 *
 * @param array  $tags Allowed tags, attributes, and/or entities.
 * @param string $context Context to judge allowed tags by. Allowed values are 'post'.
 *
 * @return array
 */
function custom_wpkses_post_tags( $tags, $context ) {

	if ( 'post' === $context ) {
		$tags['iframe'] = array(
			'src'             => true,
			'height'          => true,
			'width'           => true,
			'frameborder'     => true,
			'allowfullscreen' => true,
		);
	}

	return $tags;
}

add_filter( 'wp_kses_allowed_html', 'custom_wpkses_post_tags', 10, 2 );