The Nvidia support page does note that if you still insist on using a PC with those older operating systems, you can still connect and play on the GeForce NOW service with a supported web browser ...

After a few more tweaks, you can use the Steam Deck’s trackpad to control the mouse within the GeForce NOW web browser interface. The post GeForce NOW adds better Steam Deck support this week ...

On Windows 11/10, there are mainly two ways to use GeForce NOW to play games – using the application and browser. If you use a 32-bit version of Windows 10, you cannot use GeForce NOW.