Yes it's true! Halo: The Master Chief Collection and Reach are playable on Linux. Check out this gameplay video.
A video showing the performance differences between Proton/Wine plus DXVK and Feral Interactive's native port in Shadow of the Tomb Raider.
A single playthrough of the game No Players Online created for the game-a-month game jam.
A playthrough of the intro chapter to Digimon Story Cyber Sleuth played on Linux with Proton.
The intro sequence to Darksiders 3, played through Proton.