From the FAQs of the original post on Steam:

"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."

Wine has very tight/strict requirements for what can merged into its codebase, it's why DXVK has had to remain a standalone project because it doesn't live up to Wine's code/goal requirements.