add_filter( 'wp_kses_allowed_html', function($tags, $context) {
    if ( 'post' === $context ) {
        $tags['iframe'] = array(
            'src'             => true,
            'height'          => true,
            'width'           => true,
            'frameborder'     => true,
            'allowfullscreen' => true,
        );
    }
    return $tags;
}, 10, 2 );