Do you mean the setting called "Enable Steam Play for all titles" that was usually unchecked, that you'd have to go in and check, which some folks wouldn't do (because they might not have known they were supposed to?)

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.