Steam has now officially stopped supporting Windows 7, Windows 8, and Windows 8.1.::95.57 percent of surveyed Steam users are already on Windows 10 and 11, with nearly 2 percent of the remainder on Linux and 1.5 percent on Mac — so we may be talking about fewer than 1 percent of users on these older Windows builds. Older versions of MacOS will also lose support on February 15th, just a month and a half from now. Correction: It’s macOS 10.13 and 10.14 that are losing support. Not macOS period.
If you already have a Steam Deck, then you are basically already familiar with Linux gaming. The software-side of things (Steam, Proton, etc) is going to be the same on desktop Linux.
If a game is compatible with the Deck, then it is also comaptible with desktop.
I’ve been a Linux gamer for about a decade now. I stick with single player games, so I generally don’t have any issues, other than a minor tweak or DLL override I sometimes have to do, but that’s no different than trying to run older games on Windows.
Only real issue would be installing mods, which is possible, but could require some extra work, such as manually setting DLL overrides. I’ve had trouble getting Reloaded II to work in Linux, for example, even though they claim they support Linux.