It conflicts with existing Apple design decisions, such as Safari in full-screen with the compact tabs, or Apple Calendar in full screen mode. I wonder how that will work.

Full Screen apps do not use notch space as seen in their full screen demos

So they're not really "full screen" any more? Why even bother with the feature, and just switch it to "maximize" instead?

They are as big as pre notch. It's just now the toolbar has moved in to a space which was once black bezel. Full screen is still useful for hiding the toolbar for a distraction free view.

Except for purposes of immersion in games and movies, I might just always have the top menu bar visible if the treatment for many full screen apps is just to have an opaque black bar with no content.

I don't think Apple really designed Safari to be ordinarily used in full-screen with compact tabs.

People will get used to it... but they could have just shifted the screen down, using that chin. For example there are plenty Windows laptops with 3:2 aspect rations, where they use up all the vertical space.

They could have just moved everything down for IMO a slightly better design.

It won't be in the middle of the status bar when running anything full screen.

Some people run applications full screen most of the time. For example the browser, text editor, chat, etc.

I wonder if it automatically inserts a black bar in those situations.

It does.