A couple of days ago, Valve has released the first stable version of Proton 9.0, a compatibility layer for Windows games to run on Linux-based operating systems. The newest stable version is built on Wine 9.0 and brings all the improvements and fixes seen in earlier beta releases.
According to the release notes, the new Proton 9.0-1 enables some games that were previously playable only with Proton Experimental, including Sonic Colors: Ultimate, Command & Conquer games, The Finals, Lord of the Rings: Gollum and others, enables NVIDIA NVAPI by default for most games, adds support for Steamworks SDK 1.59, and fixes a bunch of other issues in a lot of games. The new version also limits the number of cores seen by some games in order to make them actually playable on CPUs with high core count, including Far Cry 2 and Far Cry 4, The Witcher 2: Assassins of Kings Enhanced Edition, a couple of Warhammer 40,000 games and others.