AFAIK no, and we probably never will. But we do have glimpses into it, such as this article saying Valve directly paid >100 devs to work on Linux compat:

Griffais says the company is also directly paying more than 100 open-source developers to work on the Proton compatibility layer, the Mesa graphics driver, and Vulkan, among other tasks like Steam for Linux and Chromebooks.

I would imagine they still pay outside, open source devs to work on those initiatives, though maybe not as many since they've gotten past the initial push.