Allow turning off native AMP when the URL includes a `non-amp` query string.

non-amp.php

<?php
/**
 * Allow turning off native AMP when the URL includes a `non-amp` query string.
 *
 * @package      Non_AMP
 * @author       XWP, Google, StudioPress, and contributors
 * @license      GPL-2.0+
 *
 * @wordpress-plugin
 * Plugin Name:       Non-AMP
 * Plugin URI:        https://github.com/studiopress/genesis-amp
 * Description:       Disables AMP when a 'non-amp' query string exists.
 * Version:           0.1.0
 * Author:            XWP, Google, StudioPress, and contributors
 * Author URI:        https://github.com/studiopress/genesis-amp/graphs/contributors
 * Text Domain:       genesis-amp
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * GitHub Plugin URI: https://github.com/studiopress/genesis-amp
 * Requires PHP:      5.3.6
 * Requires WP:       4.9
 */

/**
 * Hook into `amp_is_enabled` filter.  If the URL has `?non-amp`, then disable AMP.
 *
 * @param bool $is_enabled True by default.
 *
 * @return bool returns false if non-AMP is requested.
 */
add_filter( 'amp_is_enabled', function( $is_enabled ) {

	// If the URL includes `non-amp`, then disable AMP.
	if ( isset( $_GET['non-amp'] ) ) {
		return false;
	}

	// Else, return the original incoming value.
	return $is_enabled;

} );