Apparently, this is a browser extension (well, a script for a browser extension), so it works when you browse the Steam catalog through your web browser, but not through their client. Or did I miss anything?
It was the other way around. The default was to run proton-enabled games, but not random titles, unless you enabled proton for everything via the toggle ("enable for all titles") which was off by default.
Now it's on by default and the switch is gone, so it's can't inadvertently be switched off.
AI jumps out of its server farm to go refill the generator tanks?
I'm not sure about that.