https://help.steampowered.com/en/faqs/view/4784-4F2B-1321-800A#:~:text=Steam%20Support%20%3A%3A%20Windows%207%20and%20Windows%208%20Support&text=As%20of%20January%201%202024,on%20those%20versions%20of%20Windows.
"As of January 1 2024, Steam will officially stop supporting the Windows 7, Windows 8 and Windows 8.1 operating systems. After that date, the Steam Client will no longer run on those versions of Windows."
More importantly - "In order to continue running Steam and any games or other products purchased through Steam, users will need to update to a more recent version of Windows."
It’s not really Steam’s choice, they are updating their embedded browser and Chromium dropped support for older versions of Windows, so automatically Steam is forced to drop support too. The alternative is to keep in exploits and bugs that have already been fixed.
It’s a lose-lose. Keep support up, someone using Steam could get hacked and have their life ruined with no one but Valve to blame. Get rid of support, suffer the wrath of 100s of people on these old OSs who can’t just move to Linux for some reason. I know which one I would choose.