amp-remove-powered-by-wordpress-footer-credit.php

<?php
/*
 * Plugin Name: AMP Remove "Powered by WordPress" Footer Credits
 * Description: Removes this credits line for sites that don't want it.
 * Plugin URI: https://gist.github.com/westonruter/7aa079125a656fd96554d6d9123d53a4
 * Author Name: Weston Ruter
 * Author URI: https://weston.ruter.net/
 * Gist Plugin URI: https://gist.github.com/westonruter/7aa079125a656fd96554d6d9123d53a4
 */

add_filter(
	'amp_post_template_file',
	function ( $file, $template_type ) {
		if ( 'footer' === $template_type && AMP__DIR__ . '/templates/footer.php' === $file ) {
			$file = __DIR__ . '/footer.php';
		}
		return $file;
	},
	10,
	2
);

footer.php

<?php
/**
 * Footer template part.
 *
 * @package AMP
 */

/**
 * Context.
 *
 * @var AMP_Post_Template $this
 */
?>
<footer class="amp-wp-footer">
	<div>
		<h2><?php echo esc_html( wptexturize( $this->get( 'blog_name' ) ) ); ?></h2>
		<a href="#top" class="back-to-top"><?php esc_html_e( 'Back to top', 'amp' ); ?></a>
	</div>
</footer>

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