Yes, but it can't be used as a hyperlink or typed into the browser bar.

HTTP is a two-way messaging protocol. What's being talked about here is the capabilities in hyperlinks. Totally different.

That sounds like a deficiency of browsers, not of URLs. Browsers can already make arbitrary HTTP requests, and there are some rudimentary ways to expose this in hypertext (such as forms or XHR), but there’s nothing stopping a browser from letting you dump a full HTTP request into a text field and sending it.