`app.normandy.enabled` has a default setting of `true`, at least on my machine, so yeah, it’s now opt-out.

That pref doesn't matter. If you turn off the visible checkbox in settings, normandy turns off. This is true for many feature flags. You can try it yourself, I tested it - unchecking the box in settings disabled the hotfix in my user studies tab (all of them were marked as complete).

And you have to go via about:config, which is a no-go for the majority of the users.

You need about:config for that flag, but that flag isn't the only way to turn off Normandy.