A proton update? Just use the last version.

I meant mostly game updates because developers get lulled into the belief that "Proton just works, don't need to test anything". Wine and Proton developers are not a huge team either. There is no guarantee that Proton will always work. That's even spelled out in the license. There were rare occurrences of a Proton update breaking a game. Granted, they are very rare but I had to switch to an older Proton release for a game once.

As in their “linux support” will take the form of making sure they don’t break anything on their end.

Their previous Linux support consisted of "maintaining the native build across many distros" instead of targeting only Steam Linux Runtime. Of course targeting a big number of Linux distributions is more work. Valve didn't release SLR for the lulz. It's a stable environment, based on Debian Stable.