which version did robtop add demon difficulties

RobTop added the demon difficulties in Geometry Dash 2.1. The separate demon sub-difficulties were introduced with that update, not in 2.0.

Quick Scoop

• Base demon rating: added in 2.0 according to community discussion.

• Demon sub-difficulties: added in 2.1.

A simple way to think about it is: 2.0 brought demons, and 2.1 expanded them into the familiar demon tiers.

Source note

The clearest result I found was a community discussion explicitly saying the five demon difficulty levels were added in version 2.1.