Some people are getting slightly confused by the appearance of subject detection in Single point and Dynamic-area AF area modes. I think the way to think about this is upside-down from the usual way you think about subject detection. These are not go-to modes you always use, they're supplemental options for what you normally use.
The clearest example I've heard so far is this: you're photographing a single flamingo feeding in the water. When its head is up out of the water you want subject detection to find the eye and concentrate focus there. But what happens when the head is under water and unseen? In the older version of the system you'd have had to be in one of the Large-area modes or Auto-area AF. And then, you'd be subject to the whims of the system when the head disappears, which sometimes might change detection to the body, but sometimes picked an entirely different subject (particularly when you had busy foregrounds and backgrounds).
The new abilities allow you to help assert control of the focus point when subject detection isn't getting focus where you might want it, which is quite welcome.
Where people seem to be getting confused is that they think that you might always use Single point and move it to your selected subject and then subject detection takes over. Well, yes, sort of. But what happens if there are two or three flamingos? Suddenly you're going to perform a lot of focus cursor adjusting and are probably not keeping up with motion, so you still may not get focus where you want it.
This is where AF-ON+AF-area button customization can come into play. Normally, I'm in a custom wide area for focus when I'm using subject detection, but now being able to override that into a tight, small area and controlling things on the fly without losing subject detection becomes a new potentially useful option. We also got the ability to customize subject detection itself to a button, so there's an alternative approach, as well. On top of that we now have separate Recall shooting functions and Recall shooting functions (hold), which gives you two additional button customizations that might combine the above (or provide yet another option).
There's a lot to parse with these new focus options. However, nothing in my general philosophy regarding autofocus changes: autofocus as the camera wants to do it is pretty darned good these days (on all current cameras), but you will absolutely find that there are times when you need to assert control and override what the system wants to do. What Nikon has done—and I'm one of the ones that was requesting these added features—is provide more flexibility in how we manage our control process. It's going to take me some time to figure out exactly how I want my camera configured now, so it'll be a while before I update my Z9 book again. Moreover, given that there are now many more permutations in how you can assert control, it will take time to figure out which ones give you the most bang for the button press.
Short form: I don't think the appearance of subject detection in additional AF-area modes changes the primary way you work, but they offer you new ways to take control when the camera isn't doing what you want it to.