I don't think there's anything stopping you from using plain http when _not_ using a browser, such as through a server-side http client or a random python scrip

I don't think there's anything stopping you from using plain http when _not_ using a browser, such as through a server-side http client or a random python script you could whip up in 2 minutes. From what I can tell, the only enforcement is this gentleman's agreement between the browsers.