Restores the functionality of copying of HiDPI (@2x) images during S3 offload of attachments.

wpos-copy-hidpi-images.php

<?php
/*
Plugin Name: WP Offload S3 - Copy HiDPI Images
Description: Restores the functionality of copying of HiDPI (@2x) images during S3 offload of attachments.
Author: Delicious Brains
Version: 1.0
Author URI: https://deliciousbrains.com/
*/

add_filter( 'as3cf_attachment_file_paths', 'wpos3_hipdi_add_hidpi_file_paths' );

function wpos3_hipdi_add_hidpi_file_paths( $paths ) {
	global $as3cf;

	foreach ( $paths as $path ) {
		$paths[] = $as3cf->apply_file_suffix( $path, '@2x' );
	}

	return $paths;
}

add_filter( 'as3cf_find_replace_url_pairs', 'wpos3_hipdi_add_hidpi_find_replace_url_pairs', 10, 7 );

function wpos3_hipdi_add_hidpi_find_replace_url_pairs( $find_replace_pairs, $file_path, $old_url, $new_url, $meta, $old_filepath, $old_meta ) {
	global $as3cfpro;

	$file_name = basename( $file_path );

	$old_filename = $file_name;
	if ( ! is_null( $old_filepath ) ) {
		$old_filename = basename( $old_filepath );
	}

	// Replace URLs for @2x images used by most HiDPI plugins
	$hidpi_images = array();

	foreach ( $find_replace_pairs as $image ) {
		$hidpi_path = $as3cfpro->apply_file_suffix( $image['old_path'], '@2x' );
		$hidpi_file = basename( $hidpi_path );

		$old_hidpi_file = $hidpi_file;
		if ( ! is_null( $old_filepath ) ) {
			$existing_path  = str_replace( basename( $image['new_url'] ), basename( $image['old_url'] ), $image['old_path'] );
			$old_hidpi_path = $as3cfpro->apply_file_suffix( $existing_path, '@2x' );
			$old_hidpi_file = basename( $old_hidpi_path );
		}

		$hidpi_images[] = array(
			'old_url' => str_replace( $old_filename, $old_hidpi_file, $old_url ),
			'new_url' => str_replace( $file_name, $hidpi_file, $new_url ),
		);
	}

	return array_merge( $find_replace_pairs, $hidpi_images );
}

Doesn't work for me with latest version of WP Offload S3