A lot of times when a game isn't listed as 'playable' on Steam, it simply means that particular game hasn't been tested yet, and will probably still work just fine if you actually try and run it. The only real exceptions to that is games that require 'kernel level anticheat'.

Edit: Check those games out on protondb and see what that says. Since it's a 'crowdsourced' platform, it's often more up to date than Valve is.

In addition to what Wolf told you, here's a few little extra tidbits:

Some games have native Linux versions. If they don't, you typically play them through Proton, a gaming-ready version of the Wine compatibility layer. Steam directly supports this through compatibility settings (Steam -> Settings -> Compatibility for default settings or Game properties -> Compatibility for per-game settings). Sometimes specific Proton versions will be better for specific games but usually you don't need to worry about it much.

Proton is damn good. Expect performance for most games to be within ± 5% of the performance you'd get on Windows. Yes, some games run better on Proton than on native DirectX.

Valve recently decided to enable Proton by default for games that don't have a Linux version. You can enable it yourself in the settings if it isn't enabled yet.

You can even force games with a native Linux version to use Proton by setting it in the game's compatibility settings. In that case Steam will download the Windows version.

Depends, the best source for running games is https://www.protondb.com/

For games that are not on Steam, you can try Lutris.

Steam doesn't have non-Linux games enabled by default. In the settings, you'll find a compatibility tab. From there, enable the setting "Enable Steam Play for all other titles"

That's what lets it use Proton for everything by default.

Good news, it actually is and had been for a few months! https://help.steampowered.com/en/faqs/view/1B71-EDF2-EB6D-2BB3#reimage