I have an Ultra 1, so no double-tap support.
But watch app, accessibility, quick actions. Turn it on, with either full or minimal appearance,
Now a double-tap with the camera app (like the one that is supposed to be limited to the Series 9 and Ultra 2) works, with the Ultra at least. It’s not limited to the camera app. Basically any time the watch presents you with a default action, you should see it highlighted in blue, and a double-tap will be the same as tapping the default action. For notifications, the default is to dismiss them. For Fitness+ workouts, double-tap starts the workout.
So far the only strange one is the timer app, which sometimes dismisses the timer when you double-tap, but generally does not. It usually does after a few tries, though.
(h/t to @jochen : 2023 Apple Watch Models Add Double Tap Gesture - #17 by jochen)