Time flies when you're having fun I guess.

Windows 10 released on July 29, 2015 or almost 9.5 years ago.

While I'm sure there are people gaming with Steam on 7/8/8.1, I doubt it's very many (Valve's own statistics I'm sure would support this).

I would also point out that Steam on Linux only officially supports the latest LTS release of Ubuntu, which means they support old versions of Windows far longer. Of course, Ubuntu is free, and the kernel hasn't introduced any draconian hardware restrictions, but I think Valve has done a good job with their Windows support.