https://wordpress.org/support/topic/how-to-use-custom-script-in-amp-page/

amp-add-hide-content-class-to-main-menu-on-page-2906.php

<?php
/**
 * Plugin Name: AMP add hide-content class name to nav.main_menu on page with ID 2906.
 * Plugin URI: https://wordpress.org/support/topic/how-to-use-custom-script-in-amp-page/
 * Author: Weston Ruter
 */

namespace Add_Hide_Content_Class_Name_To_Main_Menu_On_Page_2906;

add_filter(
	'amp_content_sanitizers',
	function ( $sanitizers ) {
		if ( is_page( 2906 ) ) {
			require_once __DIR__ . '/class-sanitizer.php';
			$sanitizers[ __NAMESPACE__ . '\Sanitizer' ] = [];
		}
		return $sanitizers;
	}
);

class-sanitizer.php

<?php

namespace Add_Hide_Content_Class_Name_To_Main_Menu_On_Page_2906;

class Sanitizer extends \AMP_Base_Sanitizer {
	function sanitize() {
		$main_menu = $this->dom->xpath->query( '//nav[ contains( @class, "main_menu" ) ]' )->item( 0 );
		if ( $main_menu instanceof \DOMElement ) {
			$main_menu->setAttribute( 'class', $main_menu->getAttribute( 'class' ) . ' hide_content' );
		}
	}
}