I'm not aware of a way to only have the system master volume again. :(
The top item in your volume mixer screenshot is for the same "System" master volume control. You just now have the option to control the volume level for separate apps if wanted. Otherwise, you could leave the apps at 100%, and only adjust the system volume level.
The problem here is that I have to constantly reset the volume mixer because Windows keeps it changing the Discord volume level to 1 so I can't hear anyone in chat until I turn the app volume back up.
Audio ducking is one thing, but this volume mixer is more irritating than helpful.