Replaces amp query with /amp/ endpoint slug

custom-amp-endpoint.php

<?php
/**
 * Plugin Name:     Custom AMP endpoint
 * Description:     Replaces ?amp=1 with /amp/ for posts and pages
 * Author:          Pierre Gordon
 * Version:         0.1.0
 * License:         GPLv2 or later
 */

namespace Custom_AMP_Endpoint;

add_filter( 'amp_override_url', '__return_true' );

function get_amp_url( $url, $slug ) {
	return trailingslashit( $url ) . $slug;
}

function init() {
	add_filter( 'amp_url', __NAMESPACE__ . '\get_amp_url', 10, 2 );
}
add_action( 'init', __NAMESPACE__ . '\init' );

function is_amp_url( $is_amp_url, $url, $slug ) {
	return (bool) preg_match( "#/{$slug}/?$#", $url );
}
add_filter( 'is_amp_url', __NAMESPACE__ . '\is_amp_url', 10, 3 );

function set_rewrite_rule( $slug ) {
	add_rewrite_endpoint( $slug, EP_PERMALINK|EP_PAGES );
}
add_action( 'amp_set_rewrite_rule', __NAMESPACE__ . '\set_rewrite_rule' );