https://wordpress.org/support/topic/problem-with-amp-url-everything-returns-200-after-amp/

amp-infinite-endpoint-blocker.php

<?php
/**
 * Plugin Name: AMP Infinite Endpoint Blocker
 *
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Infinite Endpoint Blocker
 * Description: Block infinite URL space under amp endpoint.
 * Version:     0.1.0
 * Author:      Weston Ruter, Google
 * Author URI:  https://weston.ruter.net/
 * License:     GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

add_action(
	'template_redirect',
	function() {
		if ( ! function_exists( 'amp_get_slug' ) ) {
			return;
		}

		// Abort if query var is used, since not subject to infinite endpoint sub-path problem.
		if ( isset( $_GET[ amp_get_slug() ] ) ) {
			return;
		}

		$endpoint = get_query_var( amp_get_slug(), false );
		if ( false !== $endpoint && '1' !== (string) $endpoint && '' !== $endpoint ) {
			wp_die(
				esc_html__( 'Unrecognized AMP URL.', 'amp' ),
				esc_html__( 'Bad Request', 'amp' ),
				array( 'response' => 400 )
			);
		}

	}
);