Yep - It is delivered by the custom content plugin and is described in this topic . Until the new opacity feature was added under v2.4.3 as far as I know using the custom content plugin was the only method of adding airspace overlays.
Once this content is added a drop down is available to the top right of the map (see screen shot). This has a choice of three 'Global Airpace' layers (Airspace, Navaids and Airfields), each can be shown independently of the other. With the Opacity option used in v2.4.3 selected as there is only one layer covering all three this effectively overrides the choices made on this Global Airspace menu.
It would be fantastic if there were independent Opacity options for Airspace, Navaids and Airfields then this would do away with the need for this custom plugin completely and provide the same functionality

Many thanks
