to Use Steam Play (https://github.com/ValveSoftware/Proton) with DX11 Games it is necessary to Upgrade Mesa to the latest Version. I Added the PPA noticed in the Prerequisites: https://github.com/ValveSoftware/Proton/blob/proton_3.7/PREREQS.md But the latest version i get with this or other PPA is 18.0.5. Is there any PPA Available to use later Mesa Versions on Loki? Or do you recommend to use Juno which is based on Bionic?
I just went through this exact problem on elementary OS.
I believe adding this PPA updated my mesa drivers: https://launchpad.net/~oibaf/+archive/ubuntu/graphics-drivers/
I believe this ppa was the one that finally enabled me to updated my mesa drivers to the following version:
OpenGL version string: 4.5 (Compatibility Profile) Mesa 18.3.0-devel
Which I can confirm has allowed me to install and run Doom 2016 without issues :D