My first question was why the fork if it's open source? Then I read this part:
Modifications to Wine are submitted upstream if they're compatible with the goals and requirements of the larger Wine project; as a result, Wine users have been benefiting from parts of this work for over a year now. The rest is available as part of our source code repository for Proton and its modules.
The Git repo has a file called LICENSE.proton that is fairly minimal at the moment, but appears to establish Valve's governance over the project. So it's technically
open source because well... the source is right there on Github
, but it violates section 8 of the OSD
, and doesn't conform to any "Free Software" license as defined by the EFF. I don't see any real-world problems with either of those, but it does explain why Proton is a fork rather than simple collaboration with the Wine team.