Proton is a fork of Wine; the famous compatibility layer for Unix-like operating systems that allows running Windows applications. Proton is included inside Steam itself and is optimized for running games so that Linux gamers can directly play the games they want on Linux even if they were not originally released for Linux.

Version 9.0 of Proton was released today with the following changes according to Valve:

Previously, the following games were only playable with Proton Experimental, and now they are supported:

Dinogen Online
Photography Simulator Demo
George McGeehan Gamer Hero
The Finals
True Reporter. Mystery of Mistwood
Road to Vostok Demo
WITCH ON THE HOLY NIGHT
Lord of the Rings: Gollum
Sonic Colors: Ultimate
Command & Conquer: Red Alert 2 and Yuri’s Revenge
Command & Conquer Tiberian Sun and Firestorm
Aisling and the Tavern of Elves
Snares of Ruin 2
Insanity’s Blade
Bloody Walls

Additionally, the following games now have a limited number of cores, making them playable on high core count CPUs:

Far Cry 2
Far Cry 4
The Witcher 2: Assassins of Kings Enhanced Edition
Lara Croft and the Guardian of Light
Warhammer 40,000: Space Marine
Warhammer 40,000: Dawn of War II
Warhammer 40,000: Dawn of War II - Chaos Rising
Warhammer 40,000: Dawn of War II - Retribution
Outcast - Second Contact
Prototype

Other improvements and fixes have been made, including:

Enabling nvapi by default for most games
Ignoring system mouse acceleration in games using raw input API
Fixing security certificate warning in Brawhalla
Fixing audio issues in BOOMEROAD, DORONKO WANKO, NOTTOLOT
Fixing Civilization V Touch Only option in the launcher
Improving cutscene synchronization in Immortals Fenyx Rising
Fixing crashes in various games such as TouHou Makuka Sai, Disaster Report 4, and Super Robot Wars 30
Improving video playback in BIOMUTANT, Lords of the Fallen, Harvestella, Wayfinder, Sea of Thieves, and KING OF FIGHTERS XV
Fixing scaling for The Last Game on Steam Deck
Fixing audio issues in Airborne Kingdom and Bayonetta
Fixing minimized window issues in Escape from Monkey Island
Updating various components such as vkd3d-proton, dxvk, and dxvk-nvapi
Adding support for Steamworks SDK 1.59.

You can get the new release of Proton from Steam Play options in Steam settings:

In case you didn’t know, there is a website called protondb.com that allows you to see the current compatibility of each game on Steam with Proton so that you decide whether to purchase it or not. You can also see how other Linux users succeeded in running the games that interest you.