I'm Scottish, in my sixties, and can remember seeing brown Windsor soup on menus, almost certainly while on holiday in England with my family when I was young, so 1960s or early 1970s. I can't remember if I ever tried it though. I'm in my sixties.

A web search turns up this pre-decimalization advertisement: https://englandspuzzle.com/wp-content/uploads/2020/12/Batche... . Just plain "Windsor soup", but dark (dare I say brown?), and so clearly not the Francatelli recipe mentioned in the article. It's also on a 1926 menu in the Hartlepool Northern Daily Mail, shown here: http://www.foodsofengland.co.uk/brownwindsorsoup.htm . Binns was a department store in Sunderland, England.

So it definitely existed, but appears to have fallen out of fashion decades ago.