Should the developer manually declare Z-Ordering?

We took a look at the script from the public project Demonoire, which is provided by default.

Here, Z-Ordering is controlled directly from the event sheet.

When making an RPG style game, should Z-Ordering be declared manually?

Isn't support Construct3 Engine internally Z-Ordering?

The ability to control Z ordering is quite useful when there is an angled perspective in a game, to better control what objects go on top of or behind other ones in specific situations.

Whether you want or need to use it in your own game of course is completely up to you.