Why is that needed? What fails? What is the error?

Windows uses backslash as a directory separator, so the reference to a template may be incorrect. For example, @app/foo\bar.html.twig instead of @app/foo/bar.html.twig.

When accesing files via php, you can use both forms of slashes. It works either way. Also twig normalizes the path anyway. So again, what error are you facing?

You are right. I check again, twig really uses cache file which it has been generated by not normalized path. Won't fix.