Steam will officially stop supporting Windows 7, Windows 8, and Windows 8.1 on 1 January 2024.
In a recent patch to the Steam Client on 26 April 2023, Steam added an "End of Life" alert for Windows 7, Windows 8, and Windows 8.1. After 1 January 2024, the Steam Client will no longer operate on the mentioned systems. To be able to continue to access your library you will have to update to a more modern version of Windows.
The reason for the change is due to core features in Steam relying on an embedded version of Google Chrome. Google Chrome no longer functions on older Windows versions. Additionally future Steam versions will require Windows feature and security updates that are only present in Windows 10 and higher.