This adapter has been created to allow using a fallback filesystem for read operations when the files can't be accessed through the main adapter.
composer require jenszahner/flysystem-fallback-adapter
$main = new League\Flysystem\Adapter\AwsS3(...);
$fallback = new League\Flysystem\Adapter\Local(...);
$adapter = new Litipk\Flysystem\Fallback\FallbackAdapter($main, $fallback);