I think the apple watch fills that role now. The watch is easier to carry for activities, while reproducing the core functions of the phone (music/podcasts, payments, messages/urgent notifications), leaving little reason to check the phone at all. Also the combination of not possessing the iphone's more distracting elements, and the way that it's tiring to hold the arm upwards+inwards for an extended period probably also help keep the wearer focused on their task.

I see the difference often in the gym. The people with smart watches for music aren't getting distracted from their workouts, while the people with their phones for music tend to take long pauses, or even get caught up watching tiktok/stories/reels.