The latest stable numbered version of Valve's compatibility layer Proton has arrived. Proton 10.0-4 brings lots of fixes and new game support for SteamOS Linux.

From the changelog these are noted to now be playable:

    Surgeon Simulator: Experience Reality
    Changeling VR
    Summoners War: RUSH
    Quantum Threshold
    REACH
    Fellowship
    Metal Slug: Awakening
    The Obsessive Shadow
    Drop Dead: The Cabin
    Zero Caliber 2 Remastered
    Lost Memories 3 Side Stories
    Death by Scrolling
    Stellar Reach
    Girls' Frontline
    Modules
    Distant Worlds 2
    懒人修仙传2
    Ring Runner: Flight of the Sages
    Chronology