Tbh I have no idea what the current situation of Proton is. I thougjt it's FOSS, but most of the development is/was done by Valve employees, so people paid for their work, that they made open source, instead of hoping for volunteers.

Please correct me if I'm wrong, but I assumed the situation is still the same - most of Proton development is done by people with Valve's resources, and the general public contribution is small-ish --i.e people who do GE-Proton, or other runtimes.

But the core is work paid for by Valve. Of course, ot might live on wirhoute Valve support, it is a FOSS project, but what I understood, currently, the majority of development time is on Valve payroll.