Yes, apps WON'T cut into the notch unless the developer opts in

[zakarhino]

As per the latest developer docs, just as I expected:

On Macs that include a camera housing in the screen bezel, the system provides a compatibility mode to prevent apps from unintentionally putting content in the region the housing occupies. When this mode is active, the system changes the active area of the display to avoid the camera housing. The new active area ensures your app’s contents are always visible and not obscured by the camera housing.

So there are no concerns about backwards compatibility. Using the area above the bottom of the notch is strictly opt-in from the developer. No need to worry about old apps not working in full screen mode. We already knew this from the marketing material but it's been confirmed here again.

The new information here is that developers CAN use that screen space if they want.