It doesn't have to do anything for GNU/Linux games, that's been an option for years and it's a ghost town a-la Metal-native games. Valve (and the community) are doing the right thing by ignoring the Apple strategy of enforcing distribution terms they will abandon within the decade. Developers that want to program for Linux still can. It's just as stupid as it was when the first Steam Machine rolled out.
By supporting Proton, they are guaranteeing that modern and retro Windows games will be playable on Linux far into the future. Trying to get the next Call of Duty to support Linux natively is, quite literally, a waste of everyone's time that could possibly be involved in the process. I cannot see a single salient reason why Linux users would want developers to release a proprietary, undersupported and easily broken native build when translation can be updated and modified to support practically any runtime.