amp-suppress-eager-loading-images.php

<?php
/**
 * AMP Suppress Eager Loading Images plugin bootstrap.
 *
 * @package   Google\AMP_Suppress_Eager_Loading_Images
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Suppress Eager Loading Images
 * Plugin URI: https://gist.github.com/westonruter/e019e53d4e34235907138eae31897e46
 * Description: Prevent images (such as featured images in the Twenty Twenty-One theme) from causing AMP validation errors for the loading=eager attribute.
 * Version: 0.1
 * 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
 * Gist Plugin URI: https://gist.github.com/westonruter/e019e53d4e34235907138eae31897e46
 */

add_action(
	'wp',
	function () {
		if ( ! function_exists( 'amp_is_request' ) || ! amp_is_request() ) {
			return;
		}

		add_filter(
			'wp_get_attachment_image_attributes',
			function ( $attr ) {
				if ( isset( $attr['loading'] ) && 'eager' === $attr['loading'] ) {
					unset( $attr['loading'] );
				}
				return $attr;
			}
		);
	}
);

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c

This should be made obsolete by ampproject/amp-wp#5721.