I think they mean that you can always send back the content of a localstorage property with javascript grabbing the value and sending another request back with it in the body. Since the front end is going to run any javascript the server sends it (disregarding adblockers at least), it's sort of a more indirect version of Set-Cookie.

Yeah, that's what I meant. There's no built in support; but it's indirectly readable since client-side JS can read it.