Add page template support broken in WP 5.8

add_page_template_support_missing_in_WP_5_8.php

<?php // don't copy this line

/**
 * Add page template support
 * @return void
 * @author Karolína Vyskočilová (https://kybernaut.cz)
 * @see https://wordpress.org/support/topic/page-templates-missing-from-page-attributes-after-5-8/
 * @updated 2021-08-19
 */
 function add_page_template_support_missing_in_WP_5_8()
{
	add_theme_support('block-templates');
}
add_action('after_setup_theme', 'add_page_template_support_missing_in_WP_5_8');